immediately dropping break onto vertical/text frame doesn't always get applied
Example reproduction step:
1. Start with initial Untitled score
2. Drag & Drop a page or system break onto the initial title vertical frame
Result: break is not applied, but should be.
Bug also occurs if immediately drop a vertical/text frame and then immediately drop a page/system break ontop that frame. But for some reason, clicking around and doing other things in between may sometimes cause this bug to not happen. Also clicking the frame and then double-clicking a break element from inspector will often result in the correct desired behavior (and then even undoing that will then result in subsequent drops actually working).
Verified on master OS: Windows 10 (10.0), Arch.: x86_64, MuseScore version (64-bit): 3.0.0., revision: 3543170