How to turn ON flashlight when device screen is off?

In my app I need to start the flashlight in an activity that is started by a BroadcastReceiver. It works fine if the app is in foreground or background and the screen is ON but if the screen is OFF (with or without keyguard lock), the app crashes.

Analysis: When screen is OFF, creating the camera object still succeeds, but Camera.startPreview() throws RuntimeException.

I can understand the motiviation to forbid camera activation when screen is off, but how can I just turn on/off the flashlight?

cam = Camera.open();
//...
try {
     cam.startPreview();
}catch(RuntimeException e){
    Log.w(TAG,"Failed opening camera preview. Maybe screen locked?: " + e.getMessage());
    return;
}

Permissions:

 <uses-permission android:name="android.permission.CAMERA" />

I tried using the FLASHLIGHT (undocumented) permission, but it is ignored during runtime.

Tested on Android 8.1, Xiaomi Mi A1 minSdkVersion 22

Links I searched:

flash light is not turned on

flashlight not working,when device screen is off/sleep (android studio)

If the screen is off, the Activity won't actually be launched until its turned on. That's why the camera won't turn on. Instead, start a service to do it which can actually run immediately.