RS: concurrently delete test session of an user

2019-04-23 08:32:54,699 [ajp-nio2-8005-exec-118] ERROR OLATRuntimeException - OLAT::ERROR ^%^ N5-E617 ^%^ org.olat.core.logging
https://olat.yxz.zh.ch/auth/RepositoryEntry/253231105/assessmentToolv2/0/BusinessGroups/0/Node/99189341167922/BusinessGroup/354549779/Node/99189341167922
Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0 ^%^ **RedScreen**

<dispatchinfo>
	<componentinfo>
		<compname>courseStackPanel</compname>
		<compclass>org.olat.core.gui.components.stack.TooledStackedPanel</compclass>
		<extendedinfo>stacksize:1, active:mainBasicPanel | </extendedinfo>
		<event>
			<class&gt;org.olat.core.gui.components.stack.PopEvent</class&gt;
			<command>pop</command>
			<tostring>com:pop,org.olat.core.gui.components.stack.PopEvent@1b270</tostring>
		</event>
	</componentinfo>
	<controllerinfo>
		<controllername>org.olat.ims.qti21.ui.assessment.CorrectionIdentityListController</controllername>
		<controllername>org.olat.ims.qti21.ui.assessment.CorrectionOverviewController</controllername>
		<controllername>org.olat.course.nodes.iq.IQIdentityListCourseNodeController</controllername>
		<controllername>org.olat.course.assessment.ui.tool.AssessmentCourseTreeController</controllername>
		<controllername>org.olat.course.assessment.ui.tool.AssessmentToolController</controllername>
		<controllername>org.olat.course.run.CourseRuntimeController</controllername>
		<controllername>org.olat.core.commons.fullWebApp.BaseFullWebappController</controllername>
	</controllerinfo>
</dispatchinfo>
>>> stack of 1.cause::org.olat.core.logging.OLATRuntimeException: null
-> at org.olat.core.gui.exception.ExceptionWindowController.<init>(ExceptionWindowController.java:116)
   at org.olat.core.gui.exception.MsgFactory.createMessageChiefController(MsgFactory.java:57)
   at org.olat.core.gui.exception.MsgFactory.createMessageChiefController(MsgFactory.java:46)
   at org.olat.core.gui.components.Window.dispatchRequest(Window.java:589)
   at org.olat.core.gui.components.Window.doDispatchRequest(Window.java:282)
   at org.olat.core.gui.components.AbstractComponent.dispatchRequest(AbstractComponent.java:193)
   at org.olat.dispatcher.AuthenticatedDispatcher.processValidDispatchURI(AuthenticatedDispatcher.java:273)
   at org.olat.dispatcher.AuthenticatedDispatcher.execute(AuthenticatedDispatcher.java:190)
   at org.olat.core.servlets.OpenOLATServlet.executeUserRequest(OpenOLATServlet.java:294)
   at org.olat.core.servlets.OpenOLATServlet.doPost(OpenOLATServlet.java:237)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:661)
>>> stack of 2.cause::java.lang.NullPointerException: null
-> at org.olat.ims.qti21.ui.assessment.CorrectionIdentityListController.loadModel(CorrectionIdentityListController.java:215)
   at org.olat.ims.qti21.ui.assessment.CorrectionIdentityListController.event(CorrectionIdentityListController.java:282)
   at org.olat.core.gui.control.DefaultController.dispatchEvent(DefaultController.java:206)
   at org.olat.core.gui.components.AbstractComponent$1.run(AbstractComponent.java:240)
   at org.olat.core.logging.activity.ThreadLocalUserActivityLoggerInstaller.runWithUserActivityLogger(ThreadLocalUserActivityLoggerInstaller.java:108)
   at org.olat.core.gui.components.AbstractComponent.fireEvent(AbstractComponent.java:238)
   at org.olat.core.gui.components.stack.BreadcrumbedStackedPanel.popUpToRootController(BreadcrumbedStackedPanel.java:435)
   at org.olat.course.assessment.ui.tool.AssessmentToolController.event(AssessmentToolController.java:177)
   at org.olat.core.gui.control.DefaultController.dispatchEvent(DefaultController.java:206)
   at org.olat.core.gui.components.AbstractComponent$1.run(AbstractComponent.java:240)
   at org.olat.core.logging.activity.ThreadLocalUserActivityLoggerInstaller.runWithUserActivityLogger(ThreadLocalUserActivityLoggerInstaller.java:108)