Cannot delete a course with a date enrollment course element

Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:38.0) Gecko/20100101 Firefox/38.0 ^%^ **RedScreen**
<dispatchinfo>	
	<componentinfo>
		<compname>ffo_main_confirm_delete</compname>
		<compclass>org.olat.core.gui.components.form.flexible.impl.FormWrapperContainer</compclass>
		<extendedinfo>n/a</extendedinfo>
		<event>
			<class&gt;org.olat.core.gui.components.form.flexible.impl.FormEvent</class&gt;
			<command>done</command>
			<tostring>com:done,org.olat.core.gui.components.form.flexible.impl.FormEvent@2f23a1</tostring>
		</event>
	</componentinfo>
	<controllerinfo>
		<controllername>org.olat.repository.ui.author.ConfirmDeleteController</controllername>
		<controllername>org.olat.repository.ui.author.AuthorListController</controllername>
		<controllername>org.olat.repository.ui.author.OverviewAuthoringController</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:536)
   at org.olat.core.gui.components.Window.doDispatchRequest(Window.java:234)
   at org.olat.core.gui.components.AbstractComponent.dispatchRequest(AbstractComponent.java:193)
   at org.olat.dispatcher.AuthenticatedDispatcher.processValidDispatchURI(AuthenticatedDispatcher.java:245)
   at org.olat.dispatcher.AuthenticatedDispatcher.execute(AuthenticatedDispatcher.java:201)
   at org.olat.core.servlets.OpenOLATServlet.executeUserRequest(OpenOLATServlet.java:280)
   at org.olat.core.servlets.OpenOLATServlet.doPost(OpenOLATServlet.java:223)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
   
>>>stack of 2.cause::java.lang.NullPointerException: null
-> at org.olat.commons.calendar.ICalFileCalendarManager.getPersonalCalendar(ICalFileCalendarManager.java:676)
   at de.bps.course.nodes.den.DENManager.removeDateInUserCalendar(DENManager.java:516)
   at de.bps.course.nodes.den.DENManager.persistDENSettings(DENManager.java:258)
   at de.bps.course.nodes.DENCourseNode.cleanupOnDelete(DENCourseNode.java:157)
   at org.olat.course.CourseFactory$NodeDeletionVisitor.visit(CourseFactory.java:1063)
   at org.olat.core.util.tree.TreeVisitor.doVisit(TreeVisitor.java:70)
   at org.olat.core.util.tree.TreeVisitor.doVisit(TreeVisitor.java:67)
   at org.olat.core.util.tree.TreeVisitor.visitAll(TreeVisitor.java:57)
   at org.olat.course.CourseFactory.deleteCourse(CourseFactory.java:322)
   at org.olat.repository.handlers.CourseHandler.cleanupOnDelete(CourseHandler.java:572)
   at org.olat.repository.manager.RepositoryServiceImpl.delete(RepositoryServiceImpl.java:325)