python – If textual content enter is empty find out how to print an error message. Else find out how to go subsequent web page

class Pizza(MDApp):
def construct(self):
Window.measurement = 360,640
self.theme_cls.theme_style=”Darkish”
return Builder.load_file(‘coloration.kv’)
if title == ‘primary‘:
Pizza().run()

:
title: ‘2’
ScrollView:
size_hint_y: .9
GridLayout:
measurement:(root.width,root.top)
size_hint_y: None
size_hint_x: None
cols: 1
MDLabel:
textual content:’Full title’
TextInput:

            multiline: False
        MDLabel:
            textual content:' Cellular quantity'
        TextInput:
            multiline: False
        MDLabel:
            textual content:' Cellular quantity'
        TextInput:
            multiline: False
        MDLabel:
            textual content:'Flat,Home no'
        TextInput:
            multiline: False
        MDLabel:
            textual content:' Space,Road'
        TextInput:
            multiline: False
        MDLabel:
            textual content:' Landmark'
        TextInput:
            multiline: False
        MDLabel:
            textual content:' City/Metropolis'
        TextInput:
            multiline: False
        Widget:
        MDLabel:
            textual content:'Supply InstructionsnPlease enter your appropriate particulars nElse order will likely be invalid '
        Widget:

        Button:

            textual content: 'Submit'
            background_color: 0,0,0,0

            on_release:
                app.root.present="3"
                root.supervisor.transition.route = 'left'
            Picture:
                supply: 'photographs/login.png'

                center_x: self.mother or father.center_x
                center_y: self.mother or father.center_y