RS if a group is deleted while an other person as opened it and click on the tree (timing is really tight for this issue)

2013-09-16 15:56:58,854 [ajp-apr-8004-exec-61] ERROR OLATRuntimeException - OLAT::ERROR ^% N4-E49
Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0) ^% *RedScreen*
>>> stack of 1.cause::org.olat.core.logging.OLATRuntimeException: null
-> at org.olat.core.gui.exception.ExceptionWindowController.<init>(ExceptionWindowController.java:107)
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:508)
at org.olat.core.gui.components.Window.doDispatchRequest(Window.java:226)
at org.olat.core.gui.components.Component.dispatchRequest(Component.java:192)
at org.olat.dispatcher.AuthenticatedDispatcher.execute(AuthenticatedDispatcher.java:227)
at org.olat.core.dispatcher.DispatcherAction.execute(DispatcherAction.java:154)
at org.olat.core.servlets.OLATServlet.doPost(OLATServlet.java:155)
at org.olat.core.servlets.OLATServlet.doGet(OLATServlet.java:128)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
>>> stack of 2.cause::java.lang.NullPointerException: null
-> at org.olat.group.manager.BusinessGroupServiceImpl.setLastUsageFor(BusinessGroupServiceImpl.java:312)
at org.olat.group.ui.edit.BusinessGroupEditController.<init>(BusinessGroupEditController.java:127)
at org.olat.group.ui.run.BusinessGroupMainRunController.doAdministration(BusinessGroupMainRunController.java:709)
at org.olat.group.ui.run.BusinessGroupMainRunController.handleTreeActions(BusinessGroupMainRunController.java:662)
at org.olat.group.ui.run.BusinessGroupMainRunController.activate(BusinessGroupMainRunController.java:874)
at org.olat.group.ui.run.BusinessGroupMainRunController.activate(BusinessGroupMainRunController.java:801)
at org.olat.core.commons.fullWebApp.BaseFullWebappController.back(BaseFullWebappController.java:552)
at org.olat.core.commons.fullWebApp.BaseFullWebappController.event(BaseFullWebappController.java:516)
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)