BeginObservingDocument was already called. This means the PresShell is still
registered as an observer of the document and then later can cause a crash when
the document loops thru the observer list.
Bug 152536 r=dcone sr=jst a=jud adt=paw
git-svn-id: svn://10.0.0.236/branches/MOZILLA_1_0_BRANCH@123966 18797224-902f-48f8-a5cc-f745e15eee43