WorkbenchPage should properly handle cases where resolving the
IWorkbenchPart from CompatibilityPart.getPart() fails (in case
CompatibilityPart creation was interrupted with some runtime error).

WorkbenchPage should not propagate "null" events to part listeners and
should report parts which were in inconsistent state so that plugin
developers have a chance to find and fix the broken code.

Change-Id: Ib85601c776e9488fece0765bb44158182eb349da
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>