double-clicking on delete user throws NPE

Double-clicking on OK in the delete user (or delete group) dialog throws an NPE.

The following error is logged:

java.lang.NullPointerException
at org.hippoecm.frontend.plugins.cms.admin.users.ViewUserPanel.deleteUser(ViewUserPanel.java:153)
at org.hippoecm.frontend.plugins.cms.admin.users.ViewUserPanel.access$000(ViewUserPanel.java:52)
at org.hippoecm.frontend.plugins.cms.admin.users.ViewUserPanel$3$1.onOk(ViewUserPanel.java:134)
at org.hippoecm.frontend.dialog.AbstractDialog.handleSubmit(AbstractDialog.java:518)
at org.hippoecm.frontend.dialog.AbstractDialog$2.onSubmit(AbstractDialog.java:390)
at org.hippoecm.frontend.dialog.AbstractDialog$ButtonWrapper$1.onSubmit(AbstractDialog.java:264)
at org.apache.wicket.ajax.markup.html.form.AjaxButton$1.onSubmit(AjaxButton.java:102)
at org.apache.wicket.ajax.form.AjaxFormSubmitBehavior.onEvent(AjaxFormSubmitBehavior.java:143)
at org.apache.wicket.ajax.AjaxEventBehavior.respond(AjaxEventBehavior.java:177)
at org.apache.wicket.ajax.AbstractDefaultAjaxBehavior.onRequest(AbstractDefaultAjaxBehavior.java:299)
at org.apache.wicket.request.target.component.listener.BehaviorRequestTarget.processEvents(BehaviorRequestTarget.java:119)
at org.apache.wicket.request.AbstractRequestCycleProcessor.processEvents(AbstractRequestCycleProcessor.java:92)
at org.hippoecm.frontend.PluginRequestCycleProcessor.processEvents(PluginRequestCycleProcessor.java:31)
at org.apache.wicket.RequestCycle.processEventsAndRespond(RequestCycle.java:1250)
at org.apache.wicket.RequestCycle.step(RequestCycle.java:1329)