[Kivy] Why I have to click twice to scroll?

[Hummingbird]

I have a following problem: I need to write an app, where I will show proper answers for every question. I wrote with kivy some code and I'm struggling with one thing. I created a page. There is a button for showing answers, but after one press I only see a part of my answers and I can't scroll. But, when I press a button second time, everything is good. Could you tell me why is that? How to repair it? I would like to see all answers after pressing a button once and be able to scroll.
I attach code below:

import kivy
from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
from kivy.uix.button import Button
from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.uix.scrollview import ScrollView
from kivy.uix.boxlayout import BoxLayout
from kivy.core.window import Window
from kivy.config import Config
Config.set('graphics', 'resizable', True)
import os
import sys

class MyApp(App):
    def build(self):
        self.screen_manager = ScreenManager()

        self.answers = Answers()
        screen = Screen(name = "Answers")
        screen.add_widget(self.answers)
        self.screen_manager.add_widget(screen)

        return self.screen_manager

class Answers(GridLayout):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)

        self.rows = 3

        self.label = Label(text = "Answers: ", font_size = 40)
        self.add_widget(self.label)

        self.button = Button(text="Show answers")
        self.button.bind(on_press=self.showanswers)
        self.add_widget(self.button)

        self.scroll = ScrollableLabel(height = Window.size[1]*0.75, size_hint_y = None)
        self.add_widget(self.scroll)


    def showanswers(self, instance):
        f = open("text.txt", "r")
        lines = f.readlines()

        ScrollableLabel.update(self.scroll, lines)
        myapp.screen_manager.current = "Answers"


class ScrollableLabel(ScrollView):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.layout = GridLayout(cols = 1,  size_hint_y = None)
        self.add_widget(self.layout)

        self.lines = Label(size_hint_x = 1, size_hint_y = None, text_size = (self.width, None))

        self.scroll_to_point = Label()
        self.scroll_to_point.bind(texture_size=self.scroll_to_point.setter('size'))

        self.layout.add_widget(self.lines)
        self.layout.add_widget(self.scroll_to_point)


    def update(self, lines):
        self.lines.text = '\n'

        for i in range(len(lines)):
            self.lines.text += '\n ' +str(i+1) + ". " + lines[i]

        self.layout.height = self.lines.texture_size[1]
        self.lines.height = self.lines.texture_size[1]
        self.lines.text_size = (self.lines.width*0.75, None)

        self.scroll_to(self.scroll_to_point)

f = open("text.txt", 'a+')
for i in range(30):
    f.write("Important text \n")
f.close()

myapp = MyApp()
myapp.run()