BUG: In Selection node, get unit node only for valid MRML node references.

This fixes the issue discussed in [1]

[1] http://slicer-devel.65872.n3.nabble.com/stack-trace-for-failing-self-test-tt4031667.html

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23218 3bd1e089-480b-0410-8dfb-8563597acbee