title: Python, Kivy - QRScanner and ScreenManager
body: <p>I want to create an app with 2 (an more) screens (screen manager).
  When i make app without that QR reader, i know how to move values from one screen to another. But here i have problem with that.
  I spend 3 days on this problem and still dont have answer. Can You help me?
  Heres code:
  <a href="https://github.com/fornakter/Terminarz-Kivy-MD/blob/master/main.py" rel="nofollow noreferrer">https://github.com/fornakter/Terminarz-Kivy-MD/blob/master/main.py</a>
  Errors are on class SecoundWindow, on line 16.
  Comments explained errors wit i recive.
  Thank You.</p>