Error saving when trying to add an object to a translation document

In a 12.9 wiki, setup in multilingual (or in french monolingual), if you:

1/ Setup wiki is multilingual with french or monolingual french
2/ Go to home page
3/ Switch to french version
4/ Edit object
5/ Add an object (example GadgetClass)
6/ Try saving

Error previewdiff shows up. Exception in log is the following:

code
2020-11-12 09:12:44,986 [qtp1749186397-1801 - http://localhost:12980/xwiki/bin/preview/Main/?diff=1&version=3.2] ERROR c.x.x.XWiki - Error while evaluating velocity template [previewdiff.vm]
org.xwiki.rendering.RenderingException: Failed to execute renderer
at org.xwiki.rendering.async.internal.DefaultAsyncRendererExecutor.syncRender(DefaultAsyncRendererExecutor.java:270)
at org.xwiki.rendering.async.internal.DefaultAsyncRendererExecutor.render(DefaultAsyncRendererExecutor.java:250)
at org.xwiki.rendering.async.internal.block.DefaultBlockAsyncRendererExecutor.render(DefaultBlockAsyncRendererExecutor.java:154)
at com.xpn.xwiki.internal.template.InternalTemplateManager.render(InternalTemplateManager.java:772)
at com.xpn.xwiki.internal.template.InternalTemplateManager.renderFromSkin(InternalTemplateManager.java:745)
at com.xpn.xwiki.internal.template.InternalTemplateManager.renderFromSkin(InternalTemplateManager.java:725)
at com.xpn.xwiki.internal.template.InternalTemplateManager.render(InternalTemplateManager.java:711)
at com.xpn.xwiki.internal.template.DefaultTemplateManager.render(DefaultTemplateManager.java:78)
at com.xpn.xwiki.XWiki.evaluateTemplate(XWiki.java:2516)
at com.xpn.xwiki.web.Utils.parseTemplate(Utils.java:179)
at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:572)
at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:250)
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:707)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.eclipse.jetty.servlet.ServletHolder$NotAsyncServlet.service(ServletHolder.java:1411)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:763)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1631)
at com.xpn.xwiki.web.ActionFilter.doFilter(ActionFilter.java:122)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1618)
at org.xwiki.wysiwyg.filter.ConversionFilter.doFilter(ConversionFilter.java:106)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1618)
at org.xwiki.container.servlet.filters.internal.SetHTTPHeaderFilter.doFilter(SetHTTPHeaderFilter.java:63)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1618)
at org.xwiki.container.servlet.filters.internal.SavedRequestRestorerFilter.doFilter(SavedRequestRestorerFilter.java:208)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1618)
at org.xwiki.container.servlet.filters.internal.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:111)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1618)
at org.xwiki.resource.servlet.RoutingFilter.doFilter(RoutingFilter.java:132)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1610)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:549)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:602)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:235)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1610)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:233)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1369)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:188)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:489)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1580)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:186)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1284)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:221)
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:146)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127)
at org.eclipse.jetty.server.Server.handle(Server.java:501)
at org.eclipse.jetty.server.HttpChannel.lambda$handle$1(HttpChannel.java:383)
at org.eclipse.jetty.server.HttpChannel.dispatch(HttpChannel.java:556)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:375)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:273)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:311)
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:103)
at org.eclipse.jetty.io.ChannelEndPoint$1.run(ChannelEndPoint.java:104)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.runTask(EatWhatYouKill.java:336)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:313)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:171)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:129)
at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:375)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:806)
at org.eclipse.jetty.util.thread.QueuedThreadPool$Runner.run(QueuedThreadPool.java:938)
at java.lang.Thread.run(Thread.java:748)
Caused by: org.xwiki.rendering.RenderingException: Failed to evaluate template with id [/skins/flamingo/previewdiff.vm]
at com.xpn.xwiki.internal.template.TemplateAsyncRenderer.evaluateContent(TemplateAsyncRenderer.java:219)
at com.xpn.xwiki.internal.template.TemplateAsyncRenderer.renderVelocity(TemplateAsyncRenderer.java:180)
at com.xpn.xwiki.internal.template.TemplateAsyncRenderer.render(TemplateAsyncRenderer.java:137)
at com.xpn.xwiki.internal.template.TemplateAsyncRenderer.render(TemplateAsyncRenderer.java:53)
at org.xwiki.rendering.async.internal.DefaultAsyncRendererExecutor.lambda$syncRender$0(DefaultAsyncRendererExecutor.java:267)
at com.xpn.xwiki.internal.security.authorization.DefaultAuthorExecutor.call(DefaultAuthorExecutor.java:98)
at org.xwiki.rendering.async.internal.DefaultAsyncRendererExecutor.syncRender(DefaultAsyncRendererExecutor.java:267)
... 65 common frames omitted
Caused by: org.xwiki.velocity.XWikiVelocityException: Failed to evaluate content with namespace [/skins/flamingo/previewdiff.vm]
at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:292)
at com.xpn.xwiki.render.DefaultVelocityManager.evaluate(DefaultVelocityManager.java:316)
at com.xpn.xwiki.internal.template.VelocityTemplateEvaluator.evaluateContent(VelocityTemplateEvaluator.java:95)
at com.xpn.xwiki.internal.template.TemplateAsyncRenderer.evaluateContent(TemplateAsyncRenderer.java:217)
... 71 common frames omitted
Caused by: org.apache.velocity.exception.MethodInvocationException: Invocation of method 'mergeDocument' in class org.xwiki.store.merge.MergeScriptService threw exception java.lang.NullPointerException at /skins/flamingo/previewdiff.vm[line 24, column 42]
at org.apache.velocity.runtime.parser.node.ASTMethod.handleInvocationException(ASTMethod.java:306)
at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:233)
at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:369)
at org.apache.velocity.runtime.parser.node.ASTReference.value(ASTReference.java:701)
at org.apache.velocity.runtime.parser.node.ASTExpression.value(ASTExpression.java:72)
at org.apache.velocity.runtime.parser.node.ASTSetDirective.render(ASTSetDirective.java:240)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:423)
at org.apache.velocity.Template.merge(Template.java:358)
at org.apache.velocity.Template.merge(Template.java:262)
at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:284)
... 74 common frames omitted
Caused by: java.lang.NullPointerException: null
at org.xwiki.store.merge.MergeScriptService.mergeDocument(MergeScriptService.java:82)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.doInvoke(UberspectImpl.java:565)
at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:548)
at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:219)
... 82 common frames omitted
code

A workaround is to add the object to the english page (which is ok as objects are shared accross translations). However in monolingual french wiki this workaround might not work and therefore we are stuck.

Marking the issue as critical as it affects the home page on a french wiki.