Crash on creating a new mesocrystal

1. create new sample
2. add mesocrystal --> CRASH

segfault in MaterialsSet::materialItemFromIdentifier

Most likely appeared after discarding material from MesocrystalItem in !2676 (merged).

changed milestone to %v22

added GUI crash labels

mentioned in merge request !2679 (merged)

mentioned in commit 32bfa19e

closed with merge request !2679 (merged)