TopLayerNode can only be a visible node

Summary:
In order to find the correct top layer node, only visible nodes should be considered.
See T24131 for further information.

Test Plan: manually tested

Reviewers: MITK Reviewer Group I, O1 MITK Core, goch

Reviewed By: MITK Reviewer Group I, O1 MITK Core, goch

Subscribers: goch, onogur

Maniphest Tasks: T24131

Differential Revision: https://phabricator.mitk.org/D98