Bug 550222 - TeamAction getTargetPage() can return null

- similar to what getTargetPart() is doing, return active page
- if the "target" part is disposed, set it to null to avoid memory leaks

Change-Id: I41e2fc0491b6c9eb524ecb02cd69f7a613ff819b
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>