[processing] Fix exception when dropping model file on qgis window

Author: nyalldawson

python/plugins/processing/ProcessingPlugin.py

@@ -75,6 +75,7 @@ def handleFileDrop(self, file):
         if not file.lower().endswith('.model3'):
             return False
         self.runAlg(file)
+        return True
 
     @staticmethod
     def runAlg(file):