kivy.factory.FactoryException：未知 class

Question

class 测试（MDApp）：

def build(self):
    self.title ='Programming'
    self.theme_cls.primary_palette = "Blue"
    return Builder.load_string(
        '''

BoxLayout: 方向:'vertical' MDToolbar: 标题:'Programming' md_bg_colour:app.theme_cls.primary_color specific_text_color:1,1,1,1

MDBottomNavigationItem:
    name: 'screen 1'
    text: 'Python'
    icon: 'language-python'
    
    Image:
        id: imageView
        source: 'python.png'
        
MDBottomNavigationItem:
    name: 'screen 2'
    text: 'C++'
    icon: 'language-cpp'
    
    Image:
        id: imageView
        source: 'cp.png'
    
MDBottomNavigationItem:
    name: 'screen 3'
    text: 'Kivy'
    icon: 'android'
    
    Image:
        id: imageView
        source: 'kivy.png'
    I    
    

''')

测试运行（）