fix(MainWindow): check if last_widget is a GtkWidget before using

[GeopJr]

Turns out that checking if last_widget (weak ref) is null is not enough, we actually have to check if it's also a widget.

Turns out that using weak ref at all is segfaulty 🤷 let's just use a normal ref and hope for no memory leaks (couldn't find any in my testing, we do try to clear the ref as often as possible (after the focus has been used, when the model gets cleared))

On my side, that resulted in a CRITICAL, but it turns out it can segfault too

fix: #1059

_{When I'm in a 'why is this happening' competition and my opponent is vala}