Fix crashes when adding group while interpolating

Summary: Solves T29554. Introduced checks at some places where the active label is used, to make sure it is not nullptr.

Test Plan: Create a segmentation with content in multiple layers, interpolate between them and add a new group. There should be no crash anymore

Reviewers: O1 MITK Reviewer Group I, kislinsk

Reviewed By: O1 MITK Reviewer Group I, kislinsk

Subscribers: kislinsk

Differential Revision: https://phabricator.mitk.org/D818