Fix crash with MO rendering after loading QTAIM wfn.

Defensive programming to ensure mesh is actually loaded before
dereferencing. This isn't a full fix. Still need to find why
QTAIM is killing the existing meshes.

Change-Id: I1ccc8519498dfc1b6bcf3118ed56e0e854b5ef53
Merge topic 'FIX_qtaim-MO-crash' into master

3a837462 Fix crash with MO rendering after loading QTAIM wfn.