fix 24377: crash on create parts if tbox present

This fix is trivial and safe.  However, I note that adding a box - whether text, horizontal, or vertical - *after* creating of parts does not link.  This appears to have been deliberately unlinked some time ago - see the following:

https://github.com/musescore/MuseScore/commit/a5d1ac231bfe3a44537e1cc0ea6b6c83092d651f#diff-1b7205502c863c4426ee75767ddb1ea4L1756

I think this needs to be revisited.  Assuming this is addressed, then I will create a series of tests in the "link" folder for frame handling.