[Qt] Wrong typecast in LCL-QT

[Juha Manninen]

In r48618 I added a test for a valid typecast in QtObjectFromWidgetH.
It raises an exception but it is cought somewhere, it can be seen only
under a debugger.

Earlier when a wrong type was passed, MDIFocusFixNeeded in
TQtWidgetSet.FocusChanged used OldWidget.LCLObject which contained
garbage.

Zeljko, to reproduce the error, try opening a project without an icon.
For example the ScrollboxControls demo I sent you earlier has no icon.
Running Lazarus under debugger, there is first an exception "unable to
load icon" or similar. Then my new test + exception is triggered.
The returned variant is not even a TObject. I noticed it when I tried
to read Obj.Classname. FPC raised an exception then.

I believe the error happens in many other situations, too.

Regards,
Juha