''_cacheManager == null': is not true audio service issue fix

If anybody getting an issue in the audio_service library in flutter then this minor code may be helpful in fixing it

WillPopScope(
onWillPop: () async {
        await _audioHandler.stop();
        return true;
      },
child: Scaffold(...))

Solution 1:^[1]

I got this error because I initialized just_audio_background and audio_service package at the same time.

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Solution	Source
Solution 1	Jeremy Caney