tabbox: Makes sure we are not calling a null pointer

The desktopClient is a weak pointer and therefore can be null. This
makes sure that we are not calling a null instance of it.

It seems likely it was introduced here 45486ce5

This is a blind non-reproduced fix for the issue here:
https://errors-eval.kde.org/organizations/kde/issues/158/


(cherry picked from commit b8112386)