NPE if an E-Mail to delete is already deleted

RedScreen on pkzs 9.3.1
2014-05-20 15:52:44,765 [ajp-apr-8040-exec-5] ERROR OLATRuntimeException - OLAT::ERROR ^%^ N40-E3 ^%^ org.olat.core.logging 
Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
**RedScreen**
<dispatchinfo>
	<componentinfo>
		<compname>link_0</compname>
		<compclass>org.olat.core.gui.components.link.Link</compclass>
		<extendedinfo>n/a</extendedinfo>
		<event>
			<class&gt;org.olat.core.gui.control.Event</class&gt;
			<command>link_0</command>
			<tostring>com:link_0,org.olat.core.gui.control.Event@be46a54a</tostring>
		</event>
	</componentinfo>
	<controllerinfo>
		<controllername>org.olat.core.gui.control.generic.modal.DialogBoxController</controllername>
		<controllername>org.olat.core.util.mail.ui.MailListController</controllername>
		<controllername>org.olat.home.HomeMainController</controllername>
		<controllername>org.olat.core.commons.fullWebApp.BaseFullWebappController</controllername>
		<controllername>org.olat.core.commons.chiefcontrollers.controller.simple.SimpleBaseController</controllername>
		<controllername>org.olat.core.commons.chiefcontrollers.BaseChiefController</controllername>
	</controllerinfo>
</dispatchinfo>
>>> stack of 1.cause::org.olat.core.logging.OLATRuntimeException: null
-> at org.olat.core.gui.exception.ExceptionWindowController.<init>(ExceptionWindowController.java:115)
   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:516)
   at org.olat.core.gui.components.Window.doDispatchRequest(Window.java:228)
   at org.olat.core.gui.components.Component.dispatchRequest(Component.java:192)
   at org.olat.dispatcher.AuthenticatedDispatcher.processValidDispatchURI(AuthenticatedDispatcher.java:241)
   at org.olat.dispatcher.AuthenticatedDispatcher.execute(AuthenticatedDispatcher.java:197)
   at org.olat.core.servlets.OpenOLATServlet.executeUserRequest(OpenOLATServlet.java:261)
   at org.olat.core.servlets.OpenOLATServlet.doGet(OpenOLATServlet.java:197)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
>>> stack of 2.cause::java.lang.NullPointerException: null
-> at org.olat.core.util.mail.manager.MailManagerImpl.delete(MailManagerImpl.java:429)
   at org.olat.core.util.mail.ui.MailListController.event(MailListController.java:406)
   at org.olat.core.gui.control.DefaultController$1.run(DefaultController.java:239)
   at org.olat.core.logging.activity.ThreadLocalUserActivityLoggerInstaller.runWithUserActivityLogger(ThreadLocalUserActivityLoggerInstaller.java:108)
   at org.olat.core.gui.control.DefaultController.dispatchEvent(DefaultController.java:236)
   at org.olat.core.gui.control.DefaultController.fireEvent(DefaultController.java:184)
   at org.olat.core.gui.control.generic.modal.DialogBoxController.event(DialogBoxController.java:205)
   at org.olat.core.gui.control.DefaultController.dispatchEvent(DefaultController.java:195)
   at org.olat.core.gui.components.Component$1.run(Component.java:238)
   at org.olat.core.logging.activity.ThreadLocalUserActivityLoggerInstaller.runWithUserActivityLogger(ThreadLocalUserActivityLoggerInstaller.java:108)
   at org.olat.core.gui.components.Component.fireEvent(Component.java:236)