Steps to reproduce

1. Create a thesaurus
2. Then a microthesaurus
3. Restart the CMS
4. Create an entry in the microthesaurus
5. Try to edit it
6. You get the following error:

   java.lang.NullPointerException
       at org.ametys.plugins.repository.data.holder.impl.DataHolderHelper.commentsToSAX(DataHolderHelper.java:310)
       at org.ametys.plugins.repository.data.holder.ModelAwareDataHolder.commentsToSAX(ModelAwareDataHolder.java:422)
       at org.ametys.cms.content.ContentGenerator._saxAttributesComments(ContentGenerator.java:148)
       at org.ametys.cms.content.ContentGenerator._saxContent(ContentGenerator.java:124)
       at org.ametys.cms.content.ContentGenerator._generateContent(ContentGenerator.java:85)
       at org.ametys.cms.content.ContentGenerator.generate(ContentGenerator.java:69)
       at org.apache.cocoon.components.pipeline.AbstractProcessingPipeline.processXMLPipeline(AbstractProcessingPipeline.java:581)
       at org.apache.cocoon.components.pipeline.impl.AbstractCachingProcessingPipeline.processXMLPipeline(AbstractCachingProcessingPipeline.java:301)
       at org.apache.cocoon.components.pipeline.AbstractProcessingPipeline.process(AbstractProcessingPipeline.java:483)
       at org.apache.cocoon.components.source.impl.SitemapSource.getInputStream(SitemapSource.java:267)
       at org.ametys.core.ui.dispatcher.DispatchGenerator._dispatching(DispatchGenerator.java:155)
       at org.ametys.core.ui.dispatcher.DispatchGenerator.generate(DispatchGenerator.java:94)
       at org.apache.cocoon.components.pipeline.AbstractProcessingPipeline.processXMLPipeline(AbstractProcessingPipeline.java:581)
       at org.apache.cocoon.components.pipeline.impl.AbstractCachingProcessingPipeline.processXMLPipeline(AbstractCachingProcessingPipeline.java:301)
       at org.apache.cocoon.components.pipeline.AbstractProcessingPipeline.process(AbstractProcessingPipeline.java:483)
       at org.apache.cocoon.components.treeprocessor.sitemap.SerializeNode.invoke(SerializeNode.java:144)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:47)
       at org.apache.cocoon.components.treeprocessor.sitemap.MatchNode.invoke(MatchNode.java:108)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69)
       at org.apache.cocoon.components.treeprocessor.sitemap.PipelineNode.invoke(PipelineNode.java:143)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69)
       at org.apache.cocoon.components.treeprocessor.sitemap.PipelinesNode.invoke(PipelinesNode.java:93)
       at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:236)
       at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:178)
       at org.apache.cocoon.components.treeprocessor.TreeProcessor.process(TreeProcessor.java:254)
       at org.apache.cocoon.components.treeprocessor.sitemap.MountNode.invoke(MountNode.java:118)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:47)
       at org.apache.cocoon.components.treeprocessor.sitemap.MatchNode.invoke(MatchNode.java:108)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69)
       at org.apache.cocoon.components.treeprocessor.sitemap.PipelineNode.invoke(PipelineNode.java:143)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69)
       at org.apache.cocoon.components.treeprocessor.sitemap.PipelinesNode.invoke(PipelinesNode.java:93)
       at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:236)
       at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:178)
       at org.apache.cocoon.components.treeprocessor.TreeProcessor.process(TreeProcessor.java:254)
       at org.apache.cocoon.components.treeprocessor.sitemap.MountNode.invoke(MountNode.java:118)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:47)
       at org.apache.cocoon.components.treeprocessor.sitemap.MatchNode.invoke(MatchNode.java:108)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69)
       at org.apache.cocoon.components.treeprocessor.sitemap.PipelineNode.invoke(PipelineNode.java:143)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69)
       at org.apache.cocoon.components.treeprocessor.sitemap.PipelinesNode.invoke(PipelinesNode.java:93)
       at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:236)
       at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:178)
       at org.apache.cocoon.components.treeprocessor.TreeProcessor.process(TreeProcessor.java:254)
       at org.apache.cocoon.components.treeprocessor.sitemap.MountNode.invoke(MountNode.java:118)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:47)
       at org.apache.cocoon.components.treeprocessor.sitemap.MatchNode.invoke(MatchNode.java:108)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69)
       at org.apache.cocoon.components.treeprocessor.sitemap.PipelineNode.invoke(PipelineNode.java:143)
       at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:69)
       at org.apache.cocoon.components.treeprocessor.sitemap.PipelinesNode.invoke(PipelinesNode.java:93)
       at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:236)
       at org.apache.cocoon.components.treeprocessor.ConcreteTreeProcessor.process(ConcreteTreeProcessor.java:178)
       at org.apache.cocoon.components.treeprocessor.TreeProcessor.process(TreeProcessor.java:254)
       at org.apache.cocoon.Cocoon.process(Cocoon.java:699)
       at org.ametys.runtime.servlet.RuntimeServlet.service(RuntimeServlet.java:569)
       at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
       at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
       at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
       at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
       at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
       at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
       at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202)
       at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
       at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:526)
       at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)
       at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
       at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:678)
       at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
       at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
       at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408)
       at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
       at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:860)
       at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1587)
       at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
       at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
       at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
       at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
       at java.base/java.lang.Thread.run(Thread.java:834)