main.py

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.button import Button
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.modalview import ModalView
#from kivy.uix.textinput import TextInput
from kivy.properties import ObjectProperty, StringProperty

Builder.load_string("""
<AppContainer>
    Button:
        text: '+'
        size_hint: (0.1, 0.1)
        pos_hint: {'right': 1}
        on_press: root.add_url_modal.open_modal()
""")

class AddUrlModal(ModalView):
    def open_modal(self):
        self.open()

class AppContainer(FloatLayout):
    add_url_modal = ObjectProperty(None)

    def __init__(self, **kwargs):
        self.add_url_modal = AddUrlModal()

    def print_info(self):
        print("Hello")

class PriceApp(App):
    def build(self):
        return AppContainer()

if __name__ == '__main__':
    PriceApp().run()