Merge branch 'master' into feature-backup-manager

cura/Machines/MaterialManager.py

@@ -76,10 +76,12 @@ class MaterialManager(QObject):
 
     def initialize(self):
         # Find all materials and put them in a matrix for quick search.
-        material_metadatas = {metadata["id"]: metadata for metadata in self._container_registry.findContainersMetadata(type = "material")}
+        material_metadatas = {metadata["id"]: metadata for metadata in
+                              self._container_registry.findContainersMetadata(type = "material") if
+                              metadata.get("GUID")}
 
         self._material_group_map = dict()
-
+                
         # Map #1
         #    root_material_id -> MaterialGroup
         for material_id, material_metadata in material_metadatas.items():
@@ -93,7 +95,7 @@ class MaterialManager(QObject):
                 self._material_group_map[root_material_id].is_read_only = self._container_registry.isReadOnly(root_material_id)
             group = self._material_group_map[root_material_id]
 
-            #Store this material in the group of the appropriate root material.
+            # Store this material in the group of the appropriate root material.
             if material_id != root_material_id:
                 new_node = MaterialNode(material_metadata)
                 group.derived_material_node_list.append(new_node)