AttributeError: 'NoneType' object has no attribute 'ids', in kivymd

Question

I created a simple MDfieldwidget I am trying to reference the text but I got an AttributeError: 'NoneType' object has no attribute 'ids', in kivymd here is my main.py file (thanks in advance for your help)

from kivymd.app import MDApp
from kivy.uix.screenmanager import Screen, ScreenManager


class LoginScreen(Screen):
    pass


class SecondScreen(Screen):
    pass


class LoginApp(MDApp):
    def build(self):

        users_name = self.root.ids.users_name
        my_label = self.root.ids.my_label
        my_label.text = users_name.text

        sm = ScreenManager()
        sm.add_widget(LoginScreen(name='Screen1'))
        sm.add_widget(SecondScreen(name='Screen2'))

        return sm


LoginApp().run()

here is my .kv file

<LoginScreen>
    MDScreen:
        name: 'Screen1'
        md_bg_color: (23/255, 31/255, 40/255, 1)

        MDCard:
            size_hint: None, None
            size: 320, 400
            pos_hint: {"center_x": .5, "center_y": .5}
            orientation: 'vertical'
            padding: '8dp'
            md_bg_color: (23/255, 31/255, 47/255, 1)
            elevation: 10
            spacing: 10

            MDLabel:
                text: 'LOGIN'
                font_style: 'Button'
                font_size: 45
                halign: 'center'
                size_hint_y: None
                height: self.texture_size[1]
                theme_text_color: 'Custom'
                text_color: (1, 1, 1, 1)
                padding_y: 15

            MDTextFieldRound:
                hint_text: 'enter username'
                icon_right: 'account'
                color_active: 1, 1, 1, 1
                pos_hint: {"center_x":.5}
                size_hint_x: None
                width: 220
                font_size: 16

            # MDSeparator:
            #         height: "1dp"

            MDTextFieldRound:
                hint_text: 'enter password'
                icon_right: 'eye-off'
                color_active: 1, 1, 1, 1
                size_hint_x: None
                width: 210
                font_size: 15
                pos_hint: {"center_x": .5}
                password: True

            MDTextField:
                id: users_name
                hint_text: "What's your name?"
                helper_text: "What your friends call you.."
                helper_text_mode: "on_focus"
                pos_hint: {"center_x": .5}
                size_hint_x: None
                width: 200

            MDRectangleFlatButton:
                text: 'LOGIN'
                pos_hint: {'center_x': .5}
                size_hint_x: None
                root_button_anim: True
                on_press:
                    # change direction to to non-movable
                    root.manager.transition.direction = 'left'
                    root.manager.current = 'Screen2'
        
            Widget:
                pos_hint_y: None
                height: 30


<SecondScreen>
    MDScreen:
        md_bg_color: (23/255, 31/255, 40/255, 1)
        name: 'Screen2'
        MDCard:
            md_bg_color: (23/255, 31/255, 47/255, 1)
            padding: 10
            elevation: 10
            orientation: 'vertical'
            spacing: 25
            pos_hint: {'center_x': .5, "center_y": .5}
            size_hint: None, None
            size: 340, 400

            MDTextButton:
                text: '< Back'
                padding_x: 0
                custom_color: (244/255, 246/255, 214/255, 1)
                pos_hint: {"center_y": .1}
                root_button_anim: True
                on_press:
                    # change direction to to non-movable
                    root.manager.transition.direction = 'right'
                    root.manager.current = 'Screen1'

            MDLabel:
                id: my_label
                text: f'Hello {my_label.text}'
                # halign: 'center'
                pos_hint: {'center_x': .5}
                size_hint_y: None
                font_style: 'Body1'
                theme_text_color: 'Custom'
                text_color: (1, 1, 1, 1)

                font_size: 20
                height: self.texture_size[1]
                padding_y: 15

            Widget:
                height: 10

Answer 1

A couple problems with your code:

• You are trying to copy the text from a MDTextField to a MDLabel before the user has a chance to enter text into the MDTextField. Also, the ids have not yet been assigned in the build() method.
• Your are trying to access ids using self.root, which is the ScreenManager, but the ids are defined in the LoginScreen and SecondScreen. See the documentation.

You can solve these problems by doing the name copy when the user enters the name, and by correctly using ids. In your kv, use the text property to trigger the copy:

        MDTextField:
            id: users_name
            hint_text: "What's your name?"
            helper_text: "What your friends call you.."
            helper_text_mode: "on_focus"
            pos_hint: {"center_x": .5}
            size_hint_x: None
            width: 200
            on_text: app.set_user_name(self.text)

Then add the set_user_name() method to the App:

def set_user_name(self, name):
    my_label = self.root.get_screen('Screen2').ids.my_label
    my_label.text = name

And, of course, remove these lines from build():

    users_name = self.root.ids.users_name
    my_label = self.root.ids.my_label
    my_label.text = users_name.text

Another approach, if you do not want the erase the text that is already in the MDLabel is to use the MDRectangleFlatButton as the trigger. In that case, the kv for the MDTextField becomes:

        MDTextField:
            id: users_name
            hint_text: "What's your name?"
            helper_text: "What your friends call you.."
            helper_text_mode: "on_focus"
            pos_hint: {"center_x": .5}
            size_hint_x: None
            width: 200

which is your original code. Add the trigger for the set_user_name() method to the Button:

        MDRectangleFlatButton:
            text: 'LOGIN'
            pos_hint: {'center_x': .5}
            size_hint_x: None
            root_button_anim: True
            on_press:
                # change direction to to non-movable
                root.manager.transition.direction = 'left'
                root.manager.current = 'Screen2'
                app.set_user_name(users_name.text)  # trigger name copy

And the set_user_name() method must be adjusted to:

def set_user_name(self, name):
    my_label = self.root.get_screen('Screen2').ids.my_label
    my_label.text += name  # append name rather than over-write