Black screen on every python app after changing settings

Question

i'm new to kivy and followed a tutorial for the screenmanager with ActionBar. In the ActionBar was a button with the function "app.open_settings()" here is a link to the documentation.

I played around a bit with the settings, changed the screen rotation, clicked on apply but nothing changed.

Now when i run the python code, i see a black screen which i need to forcefully exit with Alt+F4.

Another python code i tried (in the same workspace but different main.py and .kv-file) also just shows a black screen. It worked previously. If i press Alt+Tab i can see in the small window that the program is rotated 270°, but since its just a black screen i cant change the settings back.

Any help is very appreciated. I will post the code here, but i don't know if it will help

main.py

import kivy
from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.lang import Builder
from kivy.metrics import dp, sp
from kivy.lang import Builder


class WelcomeScreen(Screen):
    pass


class FirstScreen(Screen):
    pass


class SecondScreen(Screen):
    pass


class ScreenManager(ScreenManager):
    pass


class CrimePrevention(BoxLayout):
    pass


Builder.load_file("main.kv")


class TestApp(App):
    title = 'Kivy ScreenManager & ActionBar Demo'

    def build(self):
        return CrimePrevention()


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

main.kv

#:kivy 2.1.0

<CrimePrevention>:
    orientation: "vertical"

    canvas.before:
        Color:
            rgb: .6, .6, .6
        Rectangle:
            pos: self.pos
            size: self.size
    
    SomeMenu_ActionBar:
        id: ActionBar

    ScreenManager:
        id: sm
        WelcomeScreen:
        FirstScreen:
        SecondScreen:

<SomeMenu_ActionBar@ActionBar>:

    ActionView:
        id: ActionView

        HiddenIcon_ActionPrevious:

        ActionGroup:
            id: App_ActionGroup
            mode: "spinner"
            text: "Jump to Screen"

            ActionButton:
                text: "Crime Prediction"
                on_release: app.root.ids.sm.current = 'second'
            ActionButton:
                text: "Forum"
                on_release: app.root.ids.sm.current = 'second'
            ActionButton:
                text: "Probable Suspect"
                on_release: app.root.ids.sm.current = 'second'
        
        ActionGroup:
            id: App_ActionGroup
            mode: 'spinner'
            text: 'App'

            ActionButton:
                text: 'Settings'
                on_press: app.open_settings()
            ActionButton:
                text: 'Quit'
                on_press: app.get_running_app().stop()
        
        ActionGroup:
            id: File_ActionGroup
            mode: 'spinner'
            text: 'File'

            ActionButton:
                text: 'Open'
            ActionButton:
                text: 'Save'

<HiddenIcon_ActionPrevious@ActionPrevious>:
    title: ''
    with_previous: False
    app_icon: ''
    app_icon_width: 0
    app_icon_height: 0
    size_hint_x: None
    width: len(self.title)*10

<WelcomeScreen>:
    name: 'welcome'
    Label:
        text: 'Welcome Screen'
        font_size: sp(50)

<FirstScreen>:
    name: 'first'
    Label:
        text: 'First Screen'

<SecondScreen>:
    name: 'second'
    BoxLayout:
        orientation: 'vertical'
        Label:
            text: 'Predict Crime'
            font_size: 50
        
        BoxLayout:
            Button:
                text: 'Back to Main Menu'
                font_size: 30
                on_release: app.root.ids.sm.current = 'first'
            Button:
                text: 'get random colour screen'
                font_size: 30
                on_release: app.root.ids.sm.current = 'first'