Assertion failure adding image to vertical frame via prioperty menu

Development builds crash with an assertion failure if you right-click a vertical frame and choose Add / Image. that's because the handler tries to cast the value to an HBox:

https://github.com/musescore/MuseScore/commit/dde36087140d31d7ec428499a…

Previously we got away with this because a static_cast was used, now it's toHBox. It was wrong before but didn't raise an assertion failure. As far as I can see, there's no reason for any of that, just leave it as "e".

Workaround is not use a development build :-)

As far as I can tell there is no harmful effect from this other than development builds. Unlike other assertion failures, this doesn't indicate some unknown condition we aren't going to hands correctly. The cast doesn't do anything.