Crash when opening plugin manager after adding a plugin in the folder
How to reproduce:
1. Open MuseScore
2. Open the plugin manager dialog and close it.
3. Add a new plugin in the plugin folder
4. Go back to MuseScore and open the plugin manager.
In Qt, we get the message :
ASSERT failure in QList::operator: "index out of range", file /home/steph/Qt/5.4/gcc_64/include/QtCore/qlist.h, line 486
The problem happens if we add a plugin in the folder, but not if we delete a plugin.