Fix mypy failure

cura/Settings/MachineManager.py

@@ -337,6 +337,8 @@ class MachineManager(QObject):
         # that situation will be fixed (and not occur again, since it switches it out to the preferred variant or
         # variant instead!)
         machine_node = ContainerTree.getInstance().machines[global_stack.definition.getId()]
+        if not self._global_container_stack:
+            return
         for extruder in self._global_container_stack.extruderList:
             variant_name = extruder.variant.getName()
             variant_node = machine_node.variants.get(variant_name)