Fix removing of material

CURA-6600

cura/Machines/MaterialManager.py

@@ -253,9 +253,6 @@ class MaterialManager(QObject):
         for result in results:
             container_registry.removeContainer(result.getMetaDataEntry("id", ""))
 
-    #
-    # Methods for GUI
-    #
     @pyqtSlot("QVariant", result=bool)
     def canMaterialBeRemoved(self, material_node: "MaterialNode"):
         # Check if the material is active in any extruder train. In that case, the material shouldn't be removed!
@@ -284,9 +281,6 @@ class MaterialManager(QObject):
             if container:
                 container.setName(name)
 
-    #
-    # Removes the given material.
-    #
     @pyqtSlot("QVariant")
     def removeMaterial(self, material_node: "MaterialNode") -> None:
         root_material_id = material_node.getMetaDataEntry("base_file")

cura/Machines/Models/BaseMaterialsModel.py

@@ -128,7 +128,10 @@ class BaseMaterialsModel(ListModel):
     ## This is another convenience function which is shared by all material
     #  models so it's put here to avoid having so much duplicated code.
     def _createMaterialItem(self, root_material_id, container_node):
-        metadata = CuraContainerRegistry.getInstance().findContainersMetadata(id = container_node.container_id)[0]
+        metadata_list = CuraContainerRegistry.getInstance().findContainersMetadata(id = container_node.container_id)
+        if not metadata_list:
+            return None
+        metadata = metadata_list[0]
         item = {
             "root_material_id":     root_material_id,
             "id":                   metadata["id"],

cura/Machines/Models/FavoriteMaterialsModel.py

@@ -28,7 +28,8 @@ class FavoriteMaterialsModel(BaseMaterialsModel):
                 continue
 
             item = self._createMaterialItem(root_material_id, container_node)
-            item_list.append(item)
+            if item:
+                item_list.append(item)
 
         # Sort the item list alphabetically by name
         item_list = sorted(item_list, key = lambda d: d["brand"].upper())

cura/Machines/Models/GenericMaterialsModel.py

@@ -24,11 +24,12 @@ class GenericMaterialsModel(BaseMaterialsModel):
                 continue
 
             # Only add results for generic materials
-            if container_node.getMetaDataEntry("brand").lower() != "generic":
+            if container_node.getMetaDataEntry("brand", "unknown").lower() != "generic":
                 continue
 
             item = self._createMaterialItem(root_material_id, container_node)
-            item_list.append(item)
+            if item:
+                item_list.append(item)
 
         # Sort the item list alphabetically by name
         item_list = sorted(item_list, key = lambda d: d["name"].upper())

cura/Machines/Models/MaterialBrandsModel.py

@@ -55,7 +55,8 @@ class MaterialBrandsModel(BaseMaterialsModel):
 
             # Now handle the individual materials
             item = self._createMaterialItem(root_material_id, container_node)
-            brand_group_dict[brand][material_type].append(item)
+            if item:
+                brand_group_dict[brand][material_type].append(item)
 
         # Part 2: Organize the tree into models
         #