check for window existence

Summary:
not always after componentcompleted a window is already there.
if is not there, do the same check whenthe item eventually gets a window

Test Plan: doesn't crash

Reviewers: Kirigami, ahiemstra

Reviewed By: ahiemstra

Subscribers: ahiemstra, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D28067