NPE reported is a bug that sometimes happens and was recently resolved on master JBPM-3647 it may be caused by to early accessed process definition pane. Option to workaround is to make sure that session will be loaded first and that should be the case when you login into console you should be on Task pane (Personal tasks) which already should trigger session to be loaded, just let it be for a while there to make sure session is loaded.
If that will not help, you could try to register context listener in web.xml of jbpm-gwt-console-server that will simply invoke this on application start:
StatefulKnowledgeSessionUtil.getStatefulKnowledgeSession()
HTH