Bug 551088 - DiagnosticCache throws NPE when deleting a reference

* Add Null check to AbstractCachedTree
* Handle external references in DiagnosticCache

Change-Id: If0345b264d216c2b8b4b6386142cace8fa5f6e6a
Signed-off-by: Eugen Neufeld <eneufeld@eclipsesource.com>
3 files changed