FIx failing mypy test

CURA-8083

cura/BuildVolume.py

@@ -770,6 +770,8 @@ class BuildVolume(SceneNode):
         self.rebuild()
 
     def _scaleAreas(self, result_areas: List[Polygon]) -> None:
+        if self._global_container_stack is None:
+            return
         for i, polygon in enumerate(result_areas):
             result_areas[i] = polygon.scale(
                 100.0 / max(100.0, self._global_container_stack.getProperty("material_shrinkage_percentage_xy", "value"))