Terminating app due to uncaught exception 'com.apple.coreaudio.avfaudio', reason: 'required condition is false: file != nil' WHY?

Question

I get the following error when my game crashes.

Terminating app due to uncaught exception 'com.apple.coreaudio.avfaudio', reason: 'required condition is false: file != nil'

Please can you tell me why this is happening, I think it's audio related. The way I'm setting up my audio track is via an audio node, then adding that audio node to the self as a child. At the start of the scene the audio track plays, that's where it's called.

The way I am doing my sound fx such as when the enemy gets hit, is a little different as I am making the fx in the function and not as a public variable if you see what I mean, and adding it to self as a child in the function. The function gets called every time the enemy gets hit.

What do you think is going wrong? Thanks.