Cannot delete a user if the coach replacement in not configured and user administrator deleted

2018-04-16 14:41:10,346 [ajp-nio2-8004-exec-216] ERROR OLATRuntimeException - OLAT::ERROR ^%^ N4-E391 ^%^
https://xyz.ch/auth/UserAdminSite/0/userdelete_direct/0
Mozilla/5.0 (Windows NT 6.1; PIA; Trident/7.0; rv:11.0) like Gecko
**RedScreen** org.hibernate.PropertyValueException: not-null property references a null or transient value : org.olat.basesecurity.model.GroupMembershipImpl.identity

<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.admin.user.delete.DirectDeleteController</controllername>
		<controllername>org.olat.admin.UserAdminMainController</controllername>
		<controllername>org.olat.core.commons.fullWebApp.BaseFullWebappController</controllername>
	</controllerinfo>
</dispatchinfo>
>>> stack of 1.cause::org.olat.core.logging.OLATRuntimeException: org.hibernate.PropertyValueException: not-null property references a null or transient value : org.olat.basesecurity.model.GroupMembershipImpl.identity
-> 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:289)
   at org.olat.core.servlets.OpenOLATServlet.doPost(OpenOLATServlet.java:232)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:661)
>>> stack of 2.cause::javax.persistence.PersistenceException: org.hibernate.PropertyValueException: not-null property references a null or transient value : org.olat.basesecurity.model.GroupMembershipImpl.identity
-> at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:149)
   at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:157)
   at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:164)
   at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:789)
   at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:767)
   at org.olat.basesecurity.manager.GroupDAO.addMembershipOneWay(GroupDAO.java:115)
   at org.olat.group.manager.BusinessGroupRelationDAO.addRole(BusinessGroupRelationDAO.java:93)
   at org.olat.group.manager.BusinessGroupServiceImpl.deleteUserData(BusinessGroupServiceImpl.java:195)
   at org.olat.admin.user.delete.service.UserDeletionManager.deleteIdentity(UserDeletionManager.java:279)
   at org.olat.admin.user.delete.DirectDeleteController.deleteIdentities(DirectDeleteController.java:177)
   at org.olat.admin.user.delete.DirectDeleteController.event(DirectDeleteController.java:124)
>>> stack of 3.cause::org.hibernate.PropertyValueException: not-null property references a null or transient value : org.olat.basesecurity.model.GroupMembershipImpl.identity
-> at org.hibernate.engine.internal.Nullability.checkNullability(Nullability.java:92)
   at org.hibernate.action.internal.AbstractEntityInsertAction.nullifyTransientReferencesIfNotAlready(AbstractEntityInsertAction.java:115)
   at org.hibernate.action.internal.AbstractEntityInsertAction.makeEntityManaged(AbstractEntityInsertAction.java:124)
   at org.hibernate.engine.spi.ActionQueue.addResolvedEntityInsertAction(ActionQueue.java:286)
   at org.hibernate.engine.spi.ActionQueue.addInsertAction(ActionQueue.java:261)
   at org.hibernate.engine.spi.ActionQueue.addAction(ActionQueue.java:248)
   at org.hibernate.event.internal.AbstractSaveEventListener.addInsertAction(AbstractSaveEventListener.java:326)
   at org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:275)
   at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:182)
   at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:125)
   at org.hibernate.jpa.event.internal.core.JpaPersistEventListener.saveWithGeneratedId(JpaPersistEventListener.java:67)