SoundEngine mute and unmute receive nullpoint.

[MAAARKIN]

I have a button that mute and unmute the sound.

basically i have

//if my button was pressed
if(SoundEngine.sharedEngine().isMute()) {
SoundEngine.sharedEngine().unmute();
} else {
SoundEngine.sharedEngine().mute();
}

when i start my game, and i press a lot of times, no have problem. But, when i start my game, stop and go back to unmute the sound i receive NullPointException.

on SoundEngine line 64.