Prevent KeyError when popping modified field from readed value

issue3349
review1052003
(transplanted from 3cbb5dc3f8e88bb826aec28053eefc99b56d2ef2)

tryton/gui/window/view_form/model/record.py

@@ -115,7 +115,7 @@ class Record(SignalEvent):
                 value = id2value.get(id)
                 if record and not record.destroyed and value:
                     for key in record.modified_fields:
-                        value.pop(key)
+                        value.pop(key, None)
                     record.set(value, signal=False)
         return self.group.fields.get(name, False)