Crash when doing operation that should be blocked.

[AsteriskAmpersand]

This was asked by Asterisk to be fixed later.
I opened an mrl3 file to edit, try to populate the library from the compendium.
The program crashed when I dragged the most top level material (e.g. VFX_Water_Mt__2)
into the library instead of dragging one of the files in the list of that material.
Asterisk said the program should just block it from happening instead of crashing.
-Reported by Moonklu