Fixed django exception when get data for unknown task (#3344)

* Fixed django exception when get data for unknown task

* Remove redundant code

Co-authored-by: Nikita Manovich <nikita.manovich@intel.com>

cvat/apps/engine/views.py

@@ -533,8 +533,8 @@ class TaskViewSet(auth.TaskGetQuerySetMixin, viewsets.ModelViewSet):
     )
     @action(detail=True, methods=['POST', 'GET'])
     def data(self, request, pk):
+        db_task = self.get_object() # call check_object_permissions as well
         if request.method == 'POST':
-            db_task = self.get_object() # call check_object_permissions as well
             if db_task.data:
                 return Response(data='Adding more data is not supported',
                     status=status.HTTP_400_BAD_REQUEST)
@@ -579,7 +579,6 @@ class TaskViewSet(auth.TaskGetQuerySetMixin, viewsets.ModelViewSet):
                     elif data_quality not in possible_quality_values:
                         raise ValidationError(detail='Wrong quality value')
 
-                db_task = self.get_object()
                 db_data = db_task.data
                 if not db_data:
                     raise NotFound(detail='Cannot find requested data for the task')