Make use of GetRenderWindowPart() more robust
- Do nullptr checks for optional use
- Use OPEN strategy for mandatory use
edit: In theory, GetRenderWindow(OPEN) still could return a nullptr if MITK was compiled without a render window editor. In this case most of the touched files would horribly fail anyway, as the presence of render windows is assumed. Also a user may be unlucky and close the render window editor right in the microsecond between two uses of a render window part raw pointer. This can only be solved by changing the return type to a reference counting smart pointer. All these theoretical issues never turned out to be a real problem so I chose the pragmatic way of either doing nullptr checks or simply using OPEN without an additional check.
Test Plan: Manually test views with in-app help in foreground or closed render window editor
Maniphest Tasks: T28341
Differential Revision: https://phabricator.mitk.org/D473