Fixed a crash defect in ColorRange

This pull request fixes a crash issue in ```Color Range``` node that takes down Dynamo UI. The crashing issue is discovered during the development of ```Dynamo Scheduler```, which happens to cause some inputs to ```Color Range``` to be ```null```. The original source which resulted in ```Color Range``` crash has now been fixed in the scheduler, so it is no longer possible to reproduce this crash, however, failure to get an input ```RuntimeMirror``` should never crash Dynamo. Which is what this pull request fixes.

Hi @aparajit-pratap, if you could spare an eyeball for this, that'll be nice.