From the rest of this method, it is obvious that m_topLevel can
be null in any part of the method - we are checking against it
being null in a few places in the method - both before and after
the affending lines.

Now, there is one place where the check is not applied, and
potentially calls ->window() on the null pointer.

p.s. If there are more places where kwin does clear_or_something(); return;
it could benefit from introducing on_scope_exit and similar tricks - see
Alexandrescu's 'Declarative control flow' presentation.