[backend][assetmodel] Fix initialisation

Values of variables where wrong

beat/editor/backend/assetmodel.py

@@ -78,8 +78,8 @@ class AssetModel(QStringListModel, AssetType):
 
         super(QStringListModel, self).__init__(parent)
 
-        self.__prefix_path = AssetType.Unknown
-        self.__asset_type = None
+        self.__prefix_path = None
+        self.__asset_type = AssetType.Unknown
 
     @pyqtSlot()
     def reload(self):