Open a JBossTools editor (Properties, Web XML, etc.).
Make sure, Outline view is open.
Close Outline and open again.
View fails:
java.lang.NullPointerException
at org.jboss.tools.common.model.ui.outline.XModelObjectContentOutlineProvider.createControl(XModelObjectContentOutlineProvider.java:66)
at org.jboss.tools.common.model.ui.editor.COPWrapper.createControl(EditorPartWrapper.java:316)
at org.eclipse.ui.views.contentoutline.ContentOutline.doCreatePage(ContentOutline.java:137)
at org.eclipse.ui.part.PageBookView.createPage(PageBookView.java:411)

After that, Outline view can only be revived by closing all JBossTools editors and closing/opening the view again.