Concurrent Exception when checking for extension updates in Distribution Wizard

<div class="xwikirenderingerrordescription hidden"><pre>org.xwiki.rendering.macro.MacroExecutionException: Failed to evaluate Velocity Macro for content [#if ("$!request.action" == '')
  ## Display the step.
  #set ($discard = "#getExtensionUpdaterPlan($plan)")
  #if (!$isAjaxRequest)
    ## The list of outdated/invalid extensions is paginated so we load the CSS here because the request is not AJAX.
    #set ($discard = $xwiki.ssfx.use('uicomponents/pagination/pagination.css', true))
    $services.localization.render('platform.extension.distributionWizard.extensionsStepDescription', [
      '[[', '&gt;&gt;http://platform.xwiki.org/xwiki/bin/view/Features/Applications]]',
      '[[', '&gt;&gt;http://extensions.xwiki.org/xwiki/bin/view/Extension/Wiki+Macro+Bridge+Application]]',
      '[[', '&gt;&gt;http://extensions.xwiki.org/xwiki/bin/view/Extension/Color+Theme+Application]]',
      '[[', '&gt;&gt;http://extensions.xwiki.org/xwiki/bin/view/Extension/Component+Module]]',
      '[[', '&gt;&gt;http://extensions.xwiki.org/xwiki/bin/view/Extension/Script+Module#HScriptServices]]'])

    {{html}}#displayExtensionUpdaterTrigger($plan){{/html}}

  #end
  (% class="extensionUpdater" %)(((
    #if ($plan.state == 'FINISHED')
      (% class="xHint" %)
      $services.localization.render('platform.extension.updater.lastCheckDate',
          [$xwiki.formatDate($plan.endDate)])

      {{html}}#displayExtensionUpdaterPlan($plan){{/html}}
      #set ($showCompleteStepButton = $invalid.isEmpty())
    #elseif ($plan)
      (% class="xHint" %)
      {{translation key="platform.extension.updater.loading"/}}

      {{html}}
      #displayJobProgressBar($plan)
      #displayExtensionUpdaterPlanStatus($plan)
      {{/html}}
    #end
  )))
#elseif ($request.action.startsWith('checkForUpdates'))
  #computeExtensionUpdaterPlan($plan $request.action.equals('checkForUpdatesGlobally'))
#end]
	at org.xwiki.rendering.internal.macro.velocity.VelocityMacro.evaluateString(VelocityMacro.java:131)
	at org.xwiki.rendering.internal.macro.velocity.VelocityMacro.evaluateString(VelocityMacro.java:50)
	at org.xwiki.rendering.macro.script.AbstractScriptMacro.evaluateBlock(AbstractScriptMacro.java:286)
	at org.xwiki.rendering.macro.script.AbstractScriptMacro.execute(AbstractScriptMacro.java:182)
	at org.xwiki.rendering.macro.script.AbstractScriptMacro.execute(AbstractScriptMacro.java:58)
	at org.xwiki.rendering.internal.transformation.macro.MacroTransformation.transform(MacroTransformation.java:269)
	at org.xwiki.rendering.internal.transformation.DefaultRenderingContext.transformInContext(DefaultRenderingContext.java:183)
	at org.xwiki.rendering.internal.transformation.DefaultTransformationManager.performTransformations(DefaultTransformationManager.java:95)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.transform(InternalTemplateManager.java:473)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.execute(InternalTemplateManager.java:664)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.execute(InternalTemplateManager.java:686)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.executeNoException(InternalTemplateManager.java:650)
	at com.xpn.xwiki.internal.template.DefaultTemplateManager.executeNoException(DefaultTemplateManager.java:101)
	at org.xwiki.extension.distribution.internal.job.step.AbstractDistributionStep.execute(AbstractDistributionStep.java:120)
	at org.xwiki.extension.distribution.internal.DistributionScriptService.renderCurrentStepToXHTML(DistributionScriptService.java:246)
	at org.xwiki.extension.distribution.internal.DistributionScriptService.renderCurrentStepToXHTML(DistributionScriptService.java:232)
	at sun.reflect.GeneratedMethodAccessor703.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.doInvoke(UberspectImpl.java:395)
	at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:384)
	at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:173)
	at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:280)
	at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:369)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
	at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:342)
	at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:106)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
	at org.apache.velocity.runtime.parser.node.ASTElseIfStatement.render(ASTElseIfStatement.java:92)
	at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:106)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
	at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:342)
	at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:106)
	at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:342)
	at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluateInternal(DefaultVelocityEngine.java:256)
	at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:222)
	at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:198)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.evaluateContent(InternalTemplateManager.java:725)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.render(InternalTemplateManager.java:626)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.access$500(InternalTemplateManager.java:98)
	at com.xpn.xwiki.internal.template.InternalTemplateManager$1.call(InternalTemplateManager.java:598)
	at com.xpn.xwiki.internal.template.InternalTemplateManager$1.call(InternalTemplateManager.java:594)
	at com.xpn.xwiki.internal.template.SUExecutor.call(SUExecutor.java:91)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.renderFromSkin(InternalTemplateManager.java:593)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.renderFromSkin(InternalTemplateManager.java:570)
	at com.xpn.xwiki.internal.template.InternalTemplateManager.render(InternalTemplateManager.java:556)
	at com.xpn.xwiki.internal.template.DefaultTemplateManager.render(DefaultTemplateManager.java:65)
	at com.xpn.xwiki.XWiki.evaluateTemplate(XWiki.java:1887)
	at com.xpn.xwiki.web.Utils.parseTemplate(Utils.java:180)
	at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:444)
	at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:191)
	at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:425)
	at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:228)
	at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1913)
	at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:462)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at com.xpn.xwiki.web.ActionFilter.doFilter(ActionFilter.java:115)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at org.xwiki.wysiwyg.server.filter.ConversionFilter.doFilter(ConversionFilter.java:127)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:160)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at org.xwiki.container.servlet.filters.internal.SetHTTPHeaderFilter.doFilter(SetHTTPHeaderFilter.java:63)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at com.xpn.xwiki.plugin.webdav.XWikiDavFilter.doFilter(XWikiDavFilter.java:66)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at org.xwiki.container.servlet.filters.internal.SavedRequestRestorerFilter.doFilter(SavedRequestRestorerFilter.java:208)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at org.xwiki.container.servlet.filters.internal.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:111)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at org.xwiki.resource.servlet.RoutingFilter.doFilter(RoutingFilter.java:137)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:198)
	at net.bull.javamelody.MonitoringFilter.doFilter(MonitoringFilter.java:176)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
	at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:409)
	at org.apache.coyote.ajp.AjpProcessor.process(AjpProcessor.java:200)
	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
	at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:313)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:744)
Caused by: org.xwiki.velocity.XWikiVelocityException: Failed to evaluate content with id [/templates/distribution.vm]
	at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:224)
	at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:198)
	at org.xwiki.rendering.internal.macro.velocity.VelocityMacro.evaluateString(VelocityMacro.java:124)
	... 103 more
Caused by: org.apache.velocity.exception.VelocityException: Reference evaluation threw an exception at 44:/templates/distribution.vm[line 1423, column 10]
	at org.apache.velocity.runtime.parser.node.ASTReference.evaluate(ASTReference.java:551)
	at org.apache.velocity.runtime.parser.node.ASTExpression.evaluate(ASTExpression.java:62)
	at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:85)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
	at org.apache.velocity.runtime.directive.VelocimacroProxy.render(VelocimacroProxy.java:216)
	at org.apache.velocity.runtime.directive.RuntimeMacro.render(RuntimeMacro.java:311)
	at org.apache.velocity.runtime.directive.RuntimeMacro.render(RuntimeMacro.java:230)
	at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:207)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
	at org.apache.velocity.runtime.parser.node.ASTElseIfStatement.render(ASTElseIfStatement.java:92)
	at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:106)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
	at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:87)
	at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:342)
	at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluateInternal(DefaultVelocityEngine.java:256)
	at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:222)
	... 105 more
Caused by: java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:859)
	at java.util.ArrayList$Itr.next(ArrayList.java:831)
	at java.util.AbstractCollection.toString(AbstractCollection.java:457)
	at org.xwiki.extension.job.plan.internal.DefaultExtensionPlan.toString(DefaultExtensionPlan.java:115)
	at org.apache.velocity.runtime.parser.node.ASTReference.evaluate(ASTReference.java:547)
	... 120 more
</pre></div>