BUG: the sense of the test was inverted, leading to crash in Qt5

This code is supposed to create a new quantity ui when
a new one is discovered, but had a missing not operator
so quantities weren't added until they later found via the
selection node. On Qt4 this was not a problem but did
cause a crash in Qt5.

From: Steve Pieper <pieper@isomics.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25855 3bd1e089-480b-0410-8dfb-8563597acbee