tools: do not read from invalid Unit ID

Part-of: #181

tools/level_editor/properties_view.vala

@@ -419,7 +419,8 @@ public class ActorPropertyGrid : PropertyGrid
 			}
 		}
 
-		update();
+		if (_id != GUID_ZERO)
+			update();
 	}
 
 	private void on_project_file_added_or_changed(string type, string name, uint64 size, uint64 mtime)