Whenever we try to delete a user we receive the following error:

org.apache.wicket.WicketRuntimeException: Method onLinkClicked of interface org.apache.wicket.markup.html.link.ILinkListener targeted at [UserDeleteLink [Component id = delete]] on component [UserDeleteLink [Component id = delete]] threw an exception
	at org.apache.wicket.RequestListenerInterface.internalInvoke(RequestListenerInterface.java:268)
	at org.apache.wicket.RequestListenerInterface.invoke(RequestListenerInterface.java:216)
	at org.apache.wicket.core.request.handler.ListenerInterfaceRequestHandler.invokeListener(ListenerInterfaceRequestHandler.java:241)
	at org.apache.wicket.core.request.handler.ListenerInterfaceRequestHandler.respond(ListenerInterfaceRequestHandler.java:234)
	at org.apache.wicket.request.cycle.RequestCycle$HandlerExecutor.respond(RequestCycle.java:917)
	at org.apache.wicket.request.RequestHandlerStack.execute(RequestHandlerStack.java:64)
	at org.apache.wicket.request.cycle.RequestCycle.execute(RequestCycle.java:274)
	at org.apache.wicket.request.cycle.RequestCycle.processRequest(RequestCycle.java:231)
	at org.apache.wicket.request.cycle.RequestCycle.processRequestAndDetach(RequestCycle.java:302)
	at org.apache.wicket.protocol.ws.AbstractUpgradeFilter.processRequestCycle(AbstractUpgradeFilter.java:70)
	at org.apache.wicket.protocol.http.WicketFilter.processRequest(WicketFilter.java:203)
	at org.apache.wicket.protocol.http.WicketServlet.doGet(WicketServlet.java:137)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
	at io.onedev.server.web.DefaultWicketServlet.service(DefaultWicketServlet.java:43)
	at io.onedev.server.web.DefaultWicketServlet$$EnhancerByGuice$$160129424.GUICE$TRAMPOLINE(<generated>)
	at com.google.inject.internal.InterceptorStackCallback$InterceptedMethodInvocation.proceed(InterceptorStackCallback.java:74)
	at io.onedev.server.persistence.SessionInterceptor$1.call(SessionInterceptor.java:23)
	at io.onedev.server.persistence.DefaultSessionManager.call(DefaultSessionManager.java:90)
	at io.onedev.server.persistence.SessionInterceptor.invoke(SessionInterceptor.java:18)
	at com.google.inject.internal.InterceptorStackCallback$InterceptedMethodInvocation.proceed(InterceptorStackCallback.java:75)
	at com.google.inject.internal.InterceptorStackCallback.invoke(InterceptorStackCallback.java:55)
	at io.onedev.server.web.DefaultWicketServlet$$EnhancerByGuice$$160129424.service(<generated>)
	at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:799)
	at org.eclipse.jetty.servlet.ServletHandler$ChainEnd.doFilter(ServletHandler.java:1626)
	at com.google.inject.servlet.DefaultFilterPipeline.dispatch(DefaultFilterPipeline.java:47)
	at com.google.inject.servlet.GuiceFilter.doFilter(GuiceFilter.java:133)
	at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
	at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
	at io.onedev.server.git.GoGetFilter.doFilter(GoGetFilter.java:87)
	at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
	at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
	at io.onedev.server.git.GitLfsFilter.doFilter(GitLfsFilter.java:489)
	at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
	at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
	at io.onedev.server.git.GitFilter.doFilter(GitFilter.java:382)
	at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
	at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
	at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:61)
	at org.apache.shiro.web.servlet.AdviceFilter.executeChain(AdviceFilter.java:108)
	at org.apache.shiro.web.servlet.AdviceFilter.doFilterInternal(AdviceFilter.java:137)
	at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:154)
	at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:66)
	at org.apache.shiro.web.servlet.AdviceFilter.executeChain(AdviceFilter.java:108)
	at org.apache.shiro.web.servlet.AdviceFilter.doFilterInternal(AdviceFilter.java:137)
	at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:154)
	at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:66)
	at org.apache.shiro.web.servlet.AbstractShiroFilter.executeChain(AbstractShiroFilter.java:458)
	at org.apache.shiro.web.servlet.AbstractShiroFilter$1.call(AbstractShiroFilter.java:373)
	at org.apache.shiro.subject.support.SubjectCallable.doCall(SubjectCallable.java:90)
	at org.apache.shiro.subject.support.SubjectCallable.call(SubjectCallable.java:83)
	at org.apache.shiro.subject.support.DelegatingSubject.execute(DelegatingSubject.java:387)
	at org.apache.shiro.web.servlet.AbstractShiroFilter.doFilterInternal(AbstractShiroFilter.java:370)
	at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:154)
	at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
	at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
	at io.onedev.server.jetty.DisableTraceFilter.doFilter(DisableTraceFilter.java:28)
	at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
	at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
	at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:548)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:233)
	at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1624)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:233)
	at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1434)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:188)
	at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:501)
	at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1594)
	at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:186)
	at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1349)
	at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
	at org.eclipse.jetty.server.handler.gzip.GzipHandler.handle(GzipHandler.java:763)
	at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127)
	at org.eclipse.jetty.server.Server.handle(Server.java:516)
	at org.eclipse.jetty.server.HttpChannel.lambda$handle$1(HttpChannel.java:388)
	at org.eclipse.jetty.server.HttpChannel.dispatch(HttpChannel.java:633)
	at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:380)
	at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:277)
	at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:311)
	at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:105)
	at org.eclipse.jetty.io.ChannelEndPoint$1.run(ChannelEndPoint.java:104)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.runTask(EatWhatYouKill.java:338)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:315)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:173)
	at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:131)
	at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:386)
	at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:883)
	at org.eclipse.jetty.util.thread.QueuedThreadPool$Runner.run(QueuedThreadPool.java:1034)
	at java.base/java.lang.Thread.run(Thread.java:829)
Caused by: java.lang.reflect.InvocationTargetException
	at jdk.internal.reflect.GeneratedMethodAccessor203.invoke(Unknown Source)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.apache.wicket.RequestListenerInterface.internalInvoke(RequestListenerInterface.java:258)
	... 87 more
Caused by: java.lang.RuntimeException: Error checking user reference in project 'i12'
	at io.onedev.server.entitymanager.impl.DefaultUserManager.delete(DefaultUserManager.java:167)
	at io.onedev.server.persistence.TransactionInterceptor$1.call(TransactionInterceptor.java:23)
	at io.onedev.server.persistence.DefaultTransactionManager$1.call(DefaultTransactionManager.java:72)
	at io.onedev.server.persistence.DefaultSessionManager.call(DefaultSessionManager.java:90)
	at io.onedev.server.persistence.DefaultTransactionManager.call(DefaultTransactionManager.java:60)
	at io.onedev.server.persistence.TransactionInterceptor.invoke(TransactionInterceptor.java:18)
	at io.onedev.server.web.component.user.UserDeleteLink.onClick(UserDeleteLink.java:30)
	at org.apache.wicket.markup.html.link.Link.onLinkClicked(Link.java:190)
	at jdk.internal.reflect.GeneratedMethodAccessor203.invoke(Unknown Source)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.apache.wicket.RequestListenerInterface.internalInvoke(RequestListenerInterface.java:258)
	at org.apache.wicket.RequestListenerInterface.invoke(RequestListenerInterface.java:216)
	at org.apache.wicket.core.request.handler.ListenerInterfaceRequestHandler.invokeListener(ListenerInterfaceRequestHandler.java:241)
	at org.apache.wicket.core.request.handler.ListenerInterfaceRequestHandler.respond(ListenerInterfaceRequestHandler.java:234)
	at org.apache.wicket.request.cycle.RequestCycle$HandlerExecutor.respond(RequestCycle.java:917)
	at org.apache.wicket.request.RequestHandlerStack.execute(RequestHandlerStack.java:64)
	at org.apache.wicket.request.cycle.RequestCycle.execute(RequestCycle.java:274)
	at org.apache.wicket.request.cycle.RequestCycle.processRequest(RequestCycle.java:231)
	at org.apache.wicket.request.cycle.RequestCycle.processRequestAndDetach(RequestCycle.java:302)
	at org.apache.wicket.protocol.ws.AbstractUpgradeFilter.processRequestCycle(AbstractUpgradeFilter.java:70)
	at org.apache.wicket.protocol.http.WicketFilter.processRequest(WicketFilter.java:203)
	at org.apache.wicket.protocol.http.WicketServlet.doGet(WicketServlet.java:137)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
	at io.onedev.server.web.DefaultWicketServlet.service(DefaultWicketServlet.java:43)
	at io.onedev.server.persistence.SessionInterceptor$1.call(SessionInterceptor.java:23)
	at io.onedev.server.persistence.DefaultSessionManager.call(DefaultSessionManager.java:90)
	at io.onedev.server.persistence.SessionInterceptor.invoke(SessionInterceptor.java:18)
	... 65 more
Caused by: java.lang.NullPointerException
	at io.onedev.server.model.support.issue.BoardSpec.onDeleteUser(BoardSpec.java:356)
	at io.onedev.server.model.support.issue.ProjectIssueSetting.onDeleteUser(ProjectIssueSetting.java:95)
	at io.onedev.server.entitymanager.impl.DefaultUserManager.delete(DefaultUserManager.java:162)
	... 93 more

This happens for newly created users only there for testing the delete behaviour and for users already participating in projects.