Issues with adding custom camera widget to kivy kv file

Question

Based on arnav's example I am trying to add this custom camera widget into a kv structure. Unfortunately this 'mechanism' is still not clear to me. So I have ended up with the following code. The light of the cam is active after click on the start button. However no 'video stream' is being displayed. When running just arnav's code with cv2.VideoCapture(0), the 'video stream' is displayed.

What am I doing wrong here?

qrtest.py:

from kivy.app import App
from kivy.uix.image import Image
from kivy.clock import Clock
from kivy.graphics.texture import Texture

from kivy.uix.boxlayout import BoxLayout
from kivy.core.window import Window

import cv2
import os


class KivyCamera(Image):
    def dostart(self, capture, fps, **kwargs):
        self.capture = capture
        Clock.schedule_interval(self.update, 1.0 / fps)

    def update(self, dt):
        ret, frame = self.capture.read()
        if ret:
            # convert it to texture
            buf1 = cv2.flip(frame, 0)
            buf = buf1.tostring()
            image_texture = Texture.create(
                size=(frame.shape[1], frame.shape[0]), colorfmt='bgr')
            image_texture.blit_buffer(buf, colorfmt='bgr', bufferfmt='ubyte')
            # display image from the texture
            self.texture = image_texture


capture = None

class QrtestHome(BoxLayout):

    def init_qrtest(self):
        pass

    def dostart(self, *largs):
        global capture
        capture = cv2.VideoCapture(0)
        my_camera = KivyCamera(capture=capture, fps=10, )
        my_camera.dostart(capture,10)

    def doexit(self):
        global capture

        if capture != None:
            capture.release()

        os._exit(0)


class qrtestApp(App):
    def build(self):
        Window.clearcolor = (.4,.4,.4,1)
        Window.size = (400, 300)
        homeWin = QrtestHome()
        homeWin.init_qrtest()
        return homeWin
qrtestApp().run()

and the qrtest.kv file:

<QrtestHome>:

    BoxLayout:
        orientation: "vertical"

        Label:
            height: 20
            size_hint_y: None
            text: 'Testing the camera'

        KivyCamera:
            canvas:
                Color:
                    rgb: (0, 0.6, 0.6)

                Rectangle:
                    texture: self.texture
                    pos: (50, 30)
                    size: (300, 240)


            height: 260
            id: qrcam

        BoxLayout:
            orientation: "horizontal"
            height: 20
            size_hint_y: None

            Button:
                id: butt_start
                size_hint: 0.5,1
                text: "start"
                on_press: root.dostart()

            Button:
                id: butt_exit
                text: "quit"
                size_hint: 0.5,1
                on_press: root.doexit()