BACKPORT: Regression - Author is unable to save a document after removing a link

1. Login https://cms.demo.onehippo.com/ as an admin
2. Goto https://cms.demo.onehippo.com/?path=/hippo:namespaces/hippogogreen/banner
3. Edit the namespace Banner
4. Set the field Document to Multiple and Ordered
5. Save & commit changes
6. Login https://cms.demo.onehippo.com/ as an author in an other browser
7. Goto https://cms.demo.onehippo.com/?path=/content/documents/hippogogreen/common/banners/about-go-green
8. Edit About Go Green document
9. Add link to the field Document
10. Link the field to Quick facts about Hippo (https://cms.demo.onehippo.com/?path=/content/documents/hippogogreen/about/quick-facts/quick-facts-about-hippo)
11. Save & Close About Go Green document
12. Publish (request) About Go Green document
13. Accept the request as an admin
14. Edit About Go Green document again as an author
15. Removed the just created link to Quick facts about Hippo with the red X button.
16. Save & Close About Go Green document

Expected - Author should be able to save & close the document
Actual - Author is unable to close the document, null pointer exception is thrown in the terminal and refer screenshot

[INFO] [talledLocalContainer] 15.08.2013 13:40:47 ERROR [org.apache.wicket.RequestCycle.logRuntimeException():1558]
[INFO] [talledLocalContainer] java.lang.NullPointerException
[INFO] [talledLocalContainer] at org.hippoecm.addon.workflow.StdWorkflow.invoke(StdWorkflow.java:197)
[INFO] [talledLocalContainer] at org.hippoecm.addon.workflow.MenuAction$1.onClick(MenuAction.java:41)
[INFO] [talledLocalContainer] at org.hippoecm.addon.workflow.MenuLink$1.onEvent(MenuLink.java:51)
[INFO] [talledLocalContainer] at org.apache.wicket.ajax.AjaxEventBehavior.respond(AjaxEventBehavior.java:177)
[INFO] [talledLocalContainer] at org.apache.wicket.ajax.AbstractDefaultAjaxBehavior.onRequest(AbstractDefaultAjaxBehavior.java:312)
[INFO] [talledLocalContainer] at org.apache.wicket.request.target.component.listener.BehaviorRequestTarget.processEvents(BehaviorRequestTarget.java:157)
[INFO] [talledLocalContainer] at org.apache.wicket.request.AbstractRequestCycleProcessor.processEvents(AbstractRequestCycleProcessor.java:92)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.PluginRequestCycleProcessor.processEvents(PluginRequestCycleProcessor.java:39)
[INFO] [talledLocalContainer] at org.apache.wicket.RequestCycle.processEventsAndRespond(RequestCycle.java:1279)
[INFO] [talledLocalContainer] at org.apache.wicket.RequestCycle.step(RequestCycle.java:1358)
[INFO] [talledLocalContainer] at org.apache.wicket.RequestCycle.steps(RequestCycle.java:1465)
[INFO] [talledLocalContainer] at org.apache.wicket.RequestCycle.request(RequestCycle.java:545)
[INFO] [talledLocalContainer] at org.apache.wicket.protocol.http.WicketFilter.doGet(WicketFilter.java:486)
[INFO] [talledLocalContainer] at org.apache.wicket.protocol.http.WicketFilter.doFilter(WicketFilter.java:319)
[INFO] [talledLocalContainer] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
[INFO] [talledLocalContainer] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.plugins.login.ConcurrentLoginFilter.doFilter(ConcurrentLoginFilter.java:54)
[INFO] [talledLocalContainer] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
[INFO] [talledLocalContainer] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
[INFO] [talledLocalContainer] at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
[INFO] [talledLocalContainer] at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
[INFO] [talledLocalContainer] at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
[INFO] [talledLocalContainer] at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
[INFO] [talledLocalContainer] at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:615)
[INFO] [talledLocalContainer] at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
[INFO] [talledLocalContainer] at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
[INFO] [talledLocalContainer] at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:859)
[INFO] [talledLocalContainer] at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:602)
[INFO] [talledLocalContainer] at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
[INFO] [talledLocalContainer] at java.lang.Thread.run(Thread.java:662)