How to reproduce : 

Edit a user and set it to validate step. 

This error log is present : 
Date: Tue Feb 01 11:59:53 CET 2022 (1643713193203)Thread: http-nio-10009-exec-3
Message #: 5190
Level: ERROR
NDC:
Category: org.ametys.web.workflow.SendMailToUserFunction
Message: An error occurred: unable to send mail to notify workflow change.
Location: org.apache.cocoon.util.log.SLF4JLoggerAdapter.error(SLF4JLoggerAdapter.java:46)
Thrown:
java.lang.NullPointerException
at org.ametys.web.workflow.SendMailFunction._getContentUri(SendMailFunction.java:143)
at org.ametys.web.workflow.SendMailFunction.getBodyI18nParams(SendMailFunction.java:88)
at org.ametys.cms.workflow.SendMailFunction.getMailBody(SendMailFunction.java:189)
at org.ametys.web.workflow.SendMailFunction.getMailBody(SendMailFunction.java:63)
at org.ametys.cms.workflow.SendMailFunction.execute(SendMailFunction.java:155)
at jdk.internal.reflect.GeneratedMethodAccessor106.invoke(Unknown Source)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.ametys.runtime.plugin.component.PluginsComponentManager$ComponentInvocationHandler.invoke(PluginsComponentManager.java:198)
at com.sun.proxy.$Proxy29.execute(Unknown Source)
at com.opensymphony.workflow.AbstractWorkflow.executeFunction(AbstractWorkflow.java:871)
at com.opensymphony.workflow.AbstractWorkflow.transitionWorkflow(AbstractWorkflow.java:1275)
at org.ametys.plugins.workflow.AbstractAmetysWorkflow.transitionWorkflow(AbstractAmetysWorkflow.java:212)
at com.opensymphony.workflow.AbstractWorkflow.doAction(AbstractWorkflow.java:567)
at org.ametys.plugins.workflow.AbstractAmetysWorkflow.doAction(AbstractAmetysWorkflow.java:164)
at org.ametys.cms.workflow.ContentWorkflowAction._act(ContentWorkflowAction.java:69)
at org.ametys.plugins.workflow.cocoon.AbstractWorkflowAction.act(AbstractWorkflowAction.java:71)
at org.apache.cocoon.components.treeprocessor.sitemap.ActTypeNode.invoke(ActTypeNode.java:125)
at org.apache.cocoon.components.treeprocessor.AbstractParentProcessingNode.invokeNodes(AbstractParentProcessingNode.java:47)
at org.apache.cocoon.components.treeprocessor.sitemap.ActTypeNode.invoke(ActTypeNode.java:139)
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.buildPipeline(ConcreteTreeProcessor.java:194)
at org.apache.cocoon.components.treeprocessor.TreeProcessor.buildPipeline(TreeProcessor.java:267)
at org.apache.cocoon.components.treeprocessor.sitemap.MountNode.invoke(MountNode.java:111)
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.buildPipeline(ConcreteTreeProcessor.java:194)
at org.apache.cocoon.components.treeprocessor.TreeProcessor.buildPipeline(TreeProcessor.java:267)
at org.apache.cocoon.components.source.impl.SitemapSource.init(SitemapSource.java:333)
at org.apache.cocoon.components.source.impl.SitemapSource.<init>(SitemapSource.java:215)
at org.apache.cocoon.components.source.impl.SitemapSourceFactory.getSource(SitemapSourceFactory.java:68)
at org.apache.excalibur.source.impl.SourceResolverImpl.resolveURI(SourceResolverImpl.java:208)
at org.apache.cocoon.components.CocoonComponentManager.resolveURI(CocoonComponentManager.java:558)
at org.apache.cocoon.components.CocoonComponentManager.resolveURI(CocoonComponentManager.java:558)
at org.apache.cocoon.components.CocoonComponentManager.resolveURI(CocoonComponentManager.java:558)
at org.apache.cocoon.components.CocoonComponentManager.resolveURI(CocoonComponentManager.java:558)
at org.ametys.core.ui.dispatcher.DispatchGenerator._dispatching(DispatchGenerator.java:153)
at org.ametys.core.ui.dispatcher.DispatchGenerator.generate(DispatchGenerator.java:95)
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:580)
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:490)
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$ConnectionHandler.process(AbstractProtocol.java:853)
at org.apache.tomcat.util.net.NioEndpoint$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)