Fixed T29526
Summary:
Reset renderwindow part in destructor to also cover edge cases
where the renderwindow part went invalid, but no RenderWindowPartDeactivated
was calles (e.g. happens in the scenario of the task).
Test Plan: code review, gui test (crash cannot be repoduced)
Reviewers: O1 MITK Reviewer Group I, kislinsk
Reviewed By: O1 MITK Reviewer Group I, kislinsk
Maniphest Tasks: T29526
Differential Revision: https://phabricator.mitk.org/D946