fix: routepack serializer validate bug

apps/mtm/serializers.py

@@ -164,7 +164,7 @@ class RouteSerializer(CustomModelSerializer):
         read_only_fields = EXCLUDE_FIELDS
 
     def validate(self, attrs):
-        if 'routepack' in attrs:
+        if attrs.get('routepack', None):
             attrs['material'] = attrs['routepack'].material
         if 'mgroup' in attrs and attrs['mgroup']:
             attrs['process'] = attrs['mgroup'].process