Add safeguard to avoid MxN crashes when removing node

Summary: The setGeometry() of the RenderWindowUtilityWidget gets called when a node is removed from the DataManager. It takes care of inverting the axis when necessary, but doesn't check if the geometry it's working with actually exists. This can cause crashes when removing the last visible node (see T30226). Adding a check for nullptr solves T30226.

Test Plan: Open the MxN view, load any data and remove the last visible node (either make all the other nodes invisible, or delete all but one first). Expected: no crash

Reviewers: O3 MITK Reviewer Group II, kislinsk

Reviewed By: O3 MITK Reviewer Group II, kislinsk

Differential Revision: https://phabricator.mitk.org/D870