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

nyalldawson · nyalldawson · commit 49c29e6db5b1 · 2018-07-12T12:27:15.000+10:00
(cherry-picked from 61cc8b5)
diff --git a/python/plugins/processing/ProcessingPlugin.py b/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):
