Collection in broken state! Requesting VC results in "Internal error"

This appears to have been introduced after finding an (apparent) duplicate link in a published collection. The duplicate link wasn't added manually, so that could have been a glitch as well. After removing one of the duplicate link entries pair and saving the collection, it could no longer be requested.

All of these now return an HTML page with 'Internal error' content:

Browse to https://collections.clarin.eu/details/1083
Browse to http://hdl.handle.net/11372/VC-1083
Try to edit the VC: https://collections.clarin.eu/edit/1083
curl -X 'GET' -L 'http://hdl.handle.net/11372/VC-1083' -H 'accept: application/x-cmdi+xml'

In the final case, a stacktrace is included:

<!doctype html>
<html lang="en"><head><title>HTTP Status 500 – Internal Server Error</title><style type="text/css">body {font-family:Tahoma,Arial,sans-serif;} h1, h2, h3, b {color:white;background-color:#525D76;} h1 {font-size:22px;} h2 {font-size:16px;} h3 {font-size:14px;} p {font-size:12px;} a {color:black;} .line {height:1px;background-color:#525D76;border:none;}</style></head>
<body>
<h1>HTTP Status 500 – Internal Server Error</h1>
<hr class="line" /><p><b>Type</b> Exception Report</p><p><b>Message</b> java.lang.NullPointerException</p><p><b>Description</b> The server encountered an unexpected condition that prevented it from fulfilling the request.</p><p><b>Exception</b></p>
<pre>javax.servlet.ServletException: java.lang.NullPointerException
    org.glassfish.jersey.servlet.WebComponent.serviceImpl(WebComponent.java:410)
    org.glassfish.jersey.servlet.WebComponent.service(WebComponent.java:346)
    org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:366)
    org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:319)
    org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:205)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    eu.clarin.cmdi.virtualcollectionregistry.VcrWicketFilter.doFilter(VcrWicketFilter.java:43)
    org.apache.catalina.filters.CorsFilter.handleNonCORS(CorsFilter.java:368)
    org.apache.catalina.filters.CorsFilter.doFilter(CorsFilter.java:174)
    eu.clarin.cmdi.virtualcollectionregistry.gui.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:34)
    eu.clarin.cmdi.virtualcollectionregistry.rest.PersistenceFilter.doFilter(PersistenceFilter.java:34)
    de.mpg.aai.shhaa.AuthFilter.doFilter(AuthFilter.java:96)
</pre><p><b>Root Cause</b></p><pre>java.lang.NullPointerException
    eu.clarin.cmdi.virtualcollectionregistry.service.impl.VirtualCollectionCMDICreatorImpl.createResources(VirtualCollectionCMDICreatorImpl.java:132)
    eu.clarin.cmdi.virtualcollectionregistry.service.impl.VirtualCollectionCMDICreatorImpl.createMetadataStructure(VirtualCollectionCMDICreatorImpl.java:92)
    eu.clarin.cmdi.virtualcollectionregistry.service.impl.VirtualCollectionCMDIWriterImpl.writeCMDI(VirtualCollectionCMDIWriterImpl.java:62)
    eu.clarin.cmdi.virtualcollectionregistry.service.impl.VirtualCollectionMarshallerImpl.marshalAsCMDI(VirtualCollectionMarshallerImpl.java:194)
    eu.clarin.cmdi.virtualcollectionregistry.rest.VirtualCollectionCMDIBodyWriter.writeTo(VirtualCollectionCMDIBodyWriter.java:47)
    eu.clarin.cmdi.virtualcollectionregistry.rest.VirtualCollectionCMDIBodyWriter.writeTo(VirtualCollectionCMDIBodyWriter.java:25)
    org.glassfish.jersey.message.internal.WriterInterceptorExecutor$TerminalWriterInterceptor.invokeWriteTo(WriterInterceptorExecutor.java:242)
    org.glassfish.jersey.message.internal.WriterInterceptorExecutor$TerminalWriterInterceptor.aroundWriteTo(WriterInterceptorExecutor.java:227)
    org.glassfish.jersey.message.internal.WriterInterceptorExecutor.proceed(WriterInterceptorExecutor.java:139)
    org.glassfish.jersey.server.internal.JsonWithPaddingInterceptor.aroundWriteTo(JsonWithPaddingInterceptor.java:85)
    org.glassfish.jersey.message.internal.WriterInterceptorExecutor.proceed(WriterInterceptorExecutor.java:139)
    org.glassfish.jersey.server.internal.MappableExceptionWrapperInterceptor.aroundWriteTo(MappableExceptionWrapperInterceptor.java:61)
    org.glassfish.jersey.message.internal.WriterInterceptorExecutor.proceed(WriterInterceptorExecutor.java:139)
    org.glassfish.jersey.message.internal.MessageBodyFactory.writeTo(MessageBodyFactory.java:1116)
    org.glassfish.jersey.server.ServerRuntime$Responder.writeResponse(ServerRuntime.java:638)
    org.glassfish.jersey.server.ServerRuntime$Responder.processResponse(ServerRuntime.java:371)
    org.glassfish.jersey.server.ServerRuntime$Responder.process(ServerRuntime.java:361)
    org.glassfish.jersey.server.ServerRuntime$1.run(ServerRuntime.java:256)
    org.glassfish.jersey.internal.Errors$1.call(Errors.java:248)
    org.glassfish.jersey.internal.Errors$1.call(Errors.java:244)
    org.glassfish.jersey.internal.Errors.process(Errors.java:292)
    org.glassfish.jersey.internal.Errors.process(Errors.java:274)
    org.glassfish.jersey.internal.Errors.process(Errors.java:244)
    org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:265)
    org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:232)
    org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:680)
    org.glassfish.jersey.servlet.WebComponent.serviceImpl(WebComponent.java:394)
    org.glassfish.jersey.servlet.WebComponent.service(WebComponent.java:346)
    org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:366)
    org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:319)
    org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:205)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    eu.clarin.cmdi.virtualcollectionregistry.VcrWicketFilter.doFilter(VcrWicketFilter.java:43)
    org.apache.catalina.filters.CorsFilter.handleNonCORS(CorsFilter.java:368)
    org.apache.catalina.filters.CorsFilter.doFilter(CorsFilter.java:174)
    eu.clarin.cmdi.virtualcollectionregistry.gui.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:34)
    eu.clarin.cmdi.virtualcollectionregistry.rest.PersistenceFilter.doFilter(PersistenceFilter.java:34)
    de.mpg.aai.shhaa.AuthFilter.doFilter(AuthFilter.java:96)
</pre>
<p><b>Note</b> The full stack trace of the root cause is available in the server logs.</p><hr class="line" /><h3>Apache Tomcat/8.5.70</h3>
</body>
</html>%

Server side exception when trying to access the details page:

...
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) ~[catalina.jar:8.5.70]
    at org.apache.catalina.valves.RemoteIpValve.invoke(RemoteIpValve.java:769) ~[catalina.jar:8.5.70]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:364) ~[catalina.jar:8.5.70]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:624) ~[tomcat-coyote.jar:8.5.70]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) ~[tomcat-coyote.jar:8.5.70]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:831) ~[tomcat-coyote.jar:8.5.70]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1650) ~[tomcat-coyote.jar:8.5.70]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) ~[tomcat-coyote.jar:8.5.70]
    at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) ~[tomcat-util.jar:8.5.70]
    at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) ~[tomcat-util.jar:8.5.70]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-util.jar:8.5.70]
    at java.lang.Thread.run(Thread.java:834) ~[?:?]
Caused by: java.lang.NullPointerException
    at eu.clarin.cmdi.virtualcollectionregistry.gui.pages.ReferenceLinkPanel.<init>(ReferenceLinkPanel.java:47) ~[classes/:?]
    at eu.clarin.cmdi.virtualcollectionregistry.gui.pages.VirtualCollectionDetailsPage$3.populateItem(VirtualCollectionDetailsPage.java:426) ~[classes/:?]
    at org.apache.wicket.extensions.markup.html.repeater.data.grid.AbstractDataGridView.populateItem(AbstractDataGridView.java:156) ~[wicket-extensions-7.12.0.jar:7.12.0]
    at org.apache.wicket.markup.repeater.RefreshingView$1.newItem(RefreshingView.java:114) ~[wicket-core-7.12.0.jar:7.12.0]
    at org.apache.wicket.markup.repeater.DefaultItemReuseStrategy$1.next(DefaultItemReuseStrategy.java:75) ~[wicket-core-7.12.0.jar:7.12.0]
    at org.apache.wicket.markup.repeater.DefaultItemReuseStrategy$1.next(DefaultItemReuseStrategy.java:56) ~[wicket-core-7.12.0.jar:7.12.0]
    at org.apache.wicket.markup.repeater.RefreshingView.addItems(RefreshingView.java:189) ~[wicket-core-7.12.0.jar:7.12.0]
    at org.apache.wicket.markup.repeater.RefreshingView.onPopulate(RefreshingView.java:97) ~[wicket-core-7.12.0.jar:7.12.0]
    at org.apache.wicket.markup.repeater.AbstractRepeater.onBeforeRender(AbstractRepeater.java:124) ~[wicket-core-7.12.0.jar:7.12.0]
    at org.apache.wicket.markup.repeater.AbstractPageableView.onBeforeRender(AbstractPageableView.java:115) ~[wicket-core-7.12.0.jar:7.12.0]
    at org.apache.wicket.Component.internalBeforeRender(Component.java:950) ~[wicket-core-7.12.0.jar:7.12.0]
    at org.apache.wicket.Component.beforeRender(Component.java:1018) ~[wicket-core-7.12.0.jar:7.12.0]
    at org.apache.wicket.MarkupContainer.onBeforeRenderChildren(MarkupContainer.java:1826) ~[wicket-core-7.12.0.jar:7.12.0]

clarin-eric / VirtualCollectionRegistry

Collection in broken state! Requesting VC results in "Internal error" #189