In qtmultimedia/src/plugins/multimedia/android/audio/qandroidaudiodecoder_p.h, there is a line that this. I found it since version 6.3.3.

QIODevice *m_device = nullptr;
Decoder *m_decoder;

The pointer is not initialized, so it would be better to initialize the pointer to nullptr.

QIODevice *m_device = nullptr; 
Decoder *m_decoder = nullptr;

Moreover, this is not error, in  qtmultimedia/src/plugins/multimedia/android/audio/qandroidaudiodecoder.cpp, 

{}the ** _m_decoder_ has been initialized in the constructor.

QAndroidAudioDecoder::QAndroidAudioDecoder(QAudioDecoder *parent)    
   : QPlatformAudioDecoder(parent),     
     m_decoder(new Decoder() )
{ 
//....
}