Concurrent exception when initializing core extensions

$ sh start_xwiki.sh 
Starting Jetty on port 8080 ...
2013-10-19 17:36:23.283:INFO:oejs.Server:jetty-8.1.9.v20130131
2013-10-19 17:36:23.299:INFO:oejs.NCSARequestLog:Opened /Users/vmassol/dev/xwiki/git/xwiki-enterprise/xwiki-enterprise-distribution/xwiki-enterprise-jetty/xwiki-enterprise-jetty-hsqldb/target/xwiki-enterprise-jetty-hsqldb-5.3-SNAPSHOT/data/logs/2013_10_19.request.log
2013-10-19 17:36:23.307:INFO:oejdp.ScanningAppProvider:Deployment monitor /Users/vmassol/dev/xwiki/git/xwiki-enterprise/xwiki-enterprise-distribution/xwiki-enterprise-jetty/xwiki-enterprise-jetty-hsqldb/target/xwiki-enterprise-jetty-hsqldb-5.3-SNAPSHOT/jetty/contexts at interval 10
2013-10-19 17:36:23.313:INFO:oejd.DeploymentManager:Deployable added: /Users/vmassol/dev/xwiki/git/xwiki-enterprise/xwiki-enterprise-distribution/xwiki-enterprise-jetty/xwiki-enterprise-jetty-hsqldb/target/xwiki-enterprise-jetty-hsqldb-5.3-SNAPSHOT/jetty/contexts/root.xml
2013-10-19 17:36:23.433:INFO:oejw.StandardDescriptorProcessor:NO JSP Support for /, did not find org.apache.jasper.servlet.JspServlet
2013-10-19 17:36:23.515:INFO:oejd.DeploymentManager:Deployable added: /Users/vmassol/dev/xwiki/git/xwiki-enterprise/xwiki-enterprise-distribution/xwiki-enterprise-jetty/xwiki-enterprise-jetty-hsqldb/target/xwiki-enterprise-jetty-hsqldb-5.3-SNAPSHOT/jetty/contexts/xwiki.xml
2013-10-19 17:36:28.111:INFO:oejw.StandardDescriptorProcessor:NO JSP Support for /xwiki, did not find org.apache.jasper.servlet.JspServlet
2013-10-19 17:36:31,891 [main] INFO  o.x.s.s.i.EmbeddedSolrInstance - Starting embedded Solr server... 
Exception in thread "Core extension repository updater" java.util.ConcurrentModificationException
2013-10-19 17:36:31,895 [main] INFO  o.x.s.s.i.EmbeddedSolrInstance - Using Solr home directory: data/solr 
	at java.util.Hashtable$Enumerator.next(Hashtable.java:1031)
	at org.eclipse.aether.DefaultRepositorySystemSession.copySafe(DefaultRepositorySystemSession.java:379)
	at org.eclipse.aether.DefaultRepositorySystemSession.setSystemProperties(DefaultRepositorySystemSession.java:412)
	at org.apache.maven.repository.internal.MavenRepositorySystemUtils.newSession(MavenRepositorySystemUtils.java:130)
	at org.xwiki.extension.repository.aether.internal.AetherExtensionRepositoryFactory.createRepositorySystemSession(AetherExtensionRepositoryFactory.java:89)
	at org.xwiki.extension.repository.aether.internal.AetherExtensionRepository.createRepositorySystemSession(AetherExtensionRepository.java:170)
	at org.xwiki.extension.repository.aether.internal.AetherExtensionRepository.resolveMaven(AetherExtensionRepository.java:340)
	at org.xwiki.extension.repository.aether.internal.AetherExtensionRepository.resolveMaven(AetherExtensionRepository.java:335)
	at org.xwiki.extension.repository.aether.internal.AetherExtensionRepository.resolve(AetherExtensionRepository.java:177)
	at org.xwiki.extension.repository.internal.DefaultExtensionRepositoryManager.resolve(DefaultExtensionRepositoryManager.java:166)
	at org.xwiki.extension.repository.internal.core.DefaultCoreExtensionScanner.updateExtensions(DefaultCoreExtensionScanner.java:292)
	at org.xwiki.extension.repository.internal.core.DefaultCoreExtensionRepository$1.run(DefaultCoreExtensionRepository.java:116)
	at java.lang.Thread.run(Thread.java:695)
2013-10-19 17:36:32,889 [main] WARN  o.a.s.c.SolrCore               - New index directory detected: old=null new=data/solr/./data/index/ 
2013-10-19 17:36:34,333 [main] INFO  o.x.s.s.i.EmbeddedSolrInstance - Started embedded Solr server. 
2013-10-19 17:36:36.059:INFO:oejs.AbstractConnector:Started SelectChannelConnector@0.0.0.0:8080
2013-10-19 17:36:36.060:INFO::----------------------------------
2013-10-19 17:36:36.061:INFO::Server started, you can now open http://192.168.0.17:8080/ in your browser to access your wiki.
2013-10-19 17:36:36.061:INFO::----------------------------------