Abort importing profile if we have no global container stack

You shouldn't be able to trigger it anyway, but if you do, it'll give a proper error that the import failed.

Contributes to issue CURA-2785.

plugins/LegacyProfileReader/LegacyProfileReader.py

@@ -67,6 +67,8 @@ class LegacyProfileReader(ProfileReader):
         if file_name.split(".")[-1] != "ini":
             return None
         global_container_stack = Application.getInstance().getGlobalContainerStack()
+        if not global_container_stack:
+            return None
 
         multi_extrusion = global_container_stack.getProperty("machine_extruder_count", "value") > 1
         if multi_extrusion: