Kivy AttributeError: 'NoneType' object 没有属性 'current'

Question

I'm new to Kivy and I'd really appreciate some help. I'm trying to make the button 'Manage Residents' switch screens but it comes up with the error:

self.manager.current = 'manageResidents'
AttributeError: 'NoneType' object has no attribute 'current''

Here is my code:

import sqlite3
from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.dropdown import DropDown
from kivy.base import runTouchApp
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.uix.textinput import TextInput
from kivy.properties import ObjectProperty
        
class mainMenu(Screen):
    def __init__(self, **kwargs):
        super(mainMenu, self).__init__(**kwargs)
        # Selects the first name and last name of all residents and sorts by ascending order
        db = sqlite3.connect("mainDatabase.db")    
        cursor = db.cursor()
        dropDownValues = cursor.execute("SELECT firstName, lastName FROM residents ORDER BY firstName ASC").fetchall()
        db.close()
        mainDropdown = DropDown()#Assigns the variable mainDropdown to be a dropdown menu

This is the button that should change the screen

btn = Button(text="Manage Residents", size_hint_y=None, height=44)#Creates a button at the top of the dropdown menu that switches to manage residents screen
btn.bind(on_release=lambda btn: mainDropdown.select(btn.text))
btn.bind(on_release=self.changer)

Rest of code:

        mainDropdown.add_widget(btn)
        mainButton = Button(text='Residents', size_hint=(0.25, 0.1))#Sets the name of the dropdown button, sizes it and makes it change its text to what the user has clicked on
        mainButton.bind(on_release=mainDropdown.open)
        runTouchApp(mainButton)
    def changer(self,*args):
        self.manager.current = 'manageResidents'

class manageResidents(Screen):
    def __init__(self, **kwargs):
        super(manageResidents, self).__init__(**kwargs)
        my_label1 = Label(text="You are now on manage residents screen", font_size='24dp')

class MyApp(App):
    def build(self):
        my_screenmanager = ScreenManager()
        screen1 = mainMenu(name='Main Menu')
        screen2 = manageResidents(name='Manage Residents')
        my_screenmanager.add_widget(screen1)
        my_screenmanager.add_widget(screen2)
        return my_screenmanager

if __name__ == "__main__":
    MyApp().run()here

Answer 1

Problem

There are two problems in your application.

1. In your app, it is running runTouchApp(mainButton) . Therefore, there is no attribute, 'current'
2. There is no screen name of 'manageResidents'

Solution

1. Replace runTouchApp(mainButton) with self.add_widget(mainButton)
2. Replace 'manageResidents' with 'ManageResidents'