Fix bug introduced with frame validation. closes kriskbx#57

Author: kriskbx

src/models/frame.js

@@ -63,7 +63,7 @@ class frame {
         if(!moment(this._start).isValid())
             throw `Error: Start date is not in a valid ISO date format!`;
 
-        if(!moment(this._stop).isValid())
+        if(this._stop && !moment(this._stop).isValid())
             throw `Error: Stop date is not in a valid ISO date format!`;
 
         moment.suppressDeprecationWarnings = false;