Search for instance containers with type material

CURA-4129

cura/Settings/ContainerManager.py

@@ -336,19 +336,15 @@ class ContainerManager(QObject):
         # check if this is a material container. If so, check if any material with the same GUID is being used by any
         # stacks.
         container_ids_to_check = [container_id]
-        container_results = self._container_registry.findInstanceContainers(id = container_id)
+        container_results = self._container_registry.findInstanceContainers(id = container_id, type = "material")
         if container_results:
             this_container = container_results[0]
             container_guid = this_container.getMetaDataEntry("GUID")
-            # FIXME: only material containers have GUIDs, but to make it safer, metadata/material is also checked.
-            #        but note that this is not a proper way to check whether a container is a material container.
-            #        there should be a better way to do this
-            is_material = container_guid and this_container.getMetaDataEntry("material")
-            if is_material:
-                # check all material container IDs with the same guid
-                material_containers = self._container_registry.findInstanceContainers(GUID = container_guid)
-                if material_containers:
-                    container_ids_to_check = [container.getId() for container in material_containers]
+            # check all material container IDs with the same GUID
+            material_containers = self._container_registry.findInstanceContainers(GUID = container_guid,
+                                                                                  type = "material")
+            if material_containers:
+                container_ids_to_check = [container.getId() for container in material_containers]
 
         all_stacks = self._container_registry.findContainerStacks()
         for stack in all_stacks: