BUG: Prevent crash in pqDistanceWidget

Added a NULL check for the point properties and the point property
elements to prevent a crash.

Change-Id: I850b7eb3e0d2297959b6fa9e12c2594d849749c4
Merge topic '14620_FixRulerCrash' into next

9c86b790 BUG: Prevent crash in pqDistanceWidget