[GP] Crash on layout of harmony belonging to fret diagram
Steps to reproduce:
1. Open beams-stems-ledger-lines.gp5 from
mtest/guitarpro/ in the source.
2. MuseScore crashes.
Backtrace is attached to this issue.
The cause is probably in that at layout stage Harmony calls a function which assumes that its parent is a Segment. However a Harmony can be somehow attached to FretDiagram which sets itself as a Harmony parent upon its addition.