Unable to disable Confluence KB in ServiceDesk project when label is configured

Steps to reproduce

1. Create a ServiceDesk project and Link to a knowledge base
2. Set the Search KB column to 'No' and apply any (dummy) Label
3. Disable the Confluence KB by selecting the Don't link to a knowledge base radio button
4. This will prompt a delete dialog, click Delete
5. The following message thrown upon deletion

   Unable to remove knowledge base link
   
   The Confluence space link could not be removed. Please try again.

6. Refresh the page and the Confluence KB is actually removed

The following error is thrown in atlassian-jira.log

2015-04-24 23:16:15,629 http-bio-7272-exec-5 ERROR xxxx 1396x11254x1 sit557 10.60.3.32 /rest/servicedesk/1/servicedesk/TEST/kb [common.error.jersey.ThrowableExceptionMapper] Uncaught exception thrown by REST service: null
java.lang.NullPointerException
	at com.atlassian.servicedesk.internal.feature.confluenceknowledgebase.ConfluenceKBLink$.toKBModel(ConfluenceKBLink.scala:20)
	at com.atlassian.servicedesk.internal.feature.confluenceknowledgebase.ConfluenceKnowledgeBaseStore.deleteKBLink(ConfluenceKnowledgeBaseStore.scala:59)
	at