BUG: superscript 2 in string in units label can cause crash

Confirmed crash on Qt5.7 on both windows and mac builds.
Crash occurred when looking up model index for node
combo box of units settings panel. The same code
worked in Qt4, but may have been causing undetected
memory issue. Memory corruption is strongly implicated
in the Qt5 crash because addition of debug statements would
in some cases bypass the crash.

Co-authored-by: Andras Lasso <lasso@queensu.ca>
Co-authored-by: Csaba Pinter <csaba.pinter@queensu.ca>

From: Steve Pieper <pieper@isomics.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25883 3bd1e089-480b-0410-8dfb-8563597acbee