[Kivy] AttributeError setting the label text

[gio123]

Hi All,
I receive this error message when I try to set an input text value to a label text.

self.b5.text="You wrote " + self.username.txt
AttributeError: 'LoginScreen' object has no attribute 'b5'

This is the Python code.

from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
from kivy.uix.textinput import TextInput
from kivy.uix.button import Button 

class LoginScreen(GridLayout):

    def buttonClicked(self, instance):
        self.b4.text="You wrote " + self.username.text

    def __init__(self, **kwargs):
       super(LoginScreen, self).__init__(**kwargs)
       self.cols = 1
       
       self.add_widget(Label(text="Hello"))
       self.username = TextInput(multiline=True)
       self.add_widget(self.username)
       
       b1=(Button(text="Find"))
       self.add_widget(b1)
       b1.bind(on_press=self.buttonClicked)

       b4=(Label(text="Answer:"))
       self.add_widget(b4)

       b5=(Label(text=''))
       self.add_widget(b5)

class SimpleKivy(App):
   def build(self):
       return LoginScreen()

if __name__ == "__main__":
    SimpleKivy().run()

Any help will be greatly appreciated.
Thanks in advance for your kind support.

Regards,
Giovanni