Fix crash in setMainWindow on wayland

Summary:
It's copy of KWindowSystem::setMainWindow, but it was missing
e7249deae417cdf9fb2983b06aa118c4ae4e22df:

Verify that QWindow::fromWinId returned a QWindow before using it

Foreign window support is not available on all platforms. E.g. on
Wayland this returns a nullptr. Thus we cannot use it.

Test Plan:
Crashed kwalletd5 when clicking on "Change Password" for
a GPG wallet.

Reviewers: graesslin, dfaure

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9593