Rendering TabButton without a parent causes a segfault (with FalagardWRBase renderer)

Issue #447 resolved
Martin Preisler
created an issue

Creating a TabButton without any parent, setting it as GUISheet and trying to render that will cause a segfault because Falagard Window Renderer doesn't check if a parent exists, it just gets it and tries to dereference.

Very easy to fix this but I have to postpone it for now.

FalTabButton.cpp, line 50

Reproducibility: always