连接的GridLayout对象没有使用Kivy的属性标题

GridLayout 对象在网格内生成网格。以下是我的代码的简化版本:

class SettingsPanel(GridLayout):
    def __init__(self, *args):
        GridLayout.__init__(self, cols=2);
        self.add_widget(Label(text='SubSubFoo'))
        self.add_widget(Label(text='SubSubFoo2')) 

class Panel(GridLayout):
    def __init__(self, *args):
        GridLayout.__init__(self, cols=1);
        self.add_widget(Label(text='SubFoo'))
        self.my_second_layer_grid = SubSubGrid()
        self.add_widget(self.my_second_layer_grid) 

class GUI(App):
    def build(self):
        self.title = "My app"
        layout = GridLayout(cols=2)
        layout.add_widget(Label(text='Foo'))   
        self.my_panel = SubGrid()     
        layout.add_widget(self.my_panel)
        return layout

if __name__ == '__main__':            
    GUI().run()

尽管我成功地运用了我所说的 SubGrid ,在我插入 SubSubGrid self.my_second_layer_grid = SubSubGrid()

...
BuilderException: Parser: File "C:\Users\me\AppData\Local\Continuum\anaconda3\envs\myenv\lib\site-packages\kivy\data\style.kv", line 871:
...
    869:    Label:
    870:        size_hint_y: None
>>  871:        text: root.title
    872:        text_size: self.width - 32, None
    873:        height: max(50, self.texture_size[1] + 20)
...
AttributeError: 'SettingsPanel' object has no attribute 'title'

怎么了?

Kivy在分析 Label s码?如果是,为什么它不 text 子网格

编辑: