iOS - Null Reference Exception

Hello,

I keep getting a NullReferenceException within Xcode for a few about 20 frames. The null exception is referring to a method called 'ARManager.GetTrackingQuality'.

This is the method this;

 public int GetTrackingQuality(){
    return 100;
 }

The error;

NullReferenceException: A null value was found where an object instance was required. at ARManager.GetTrackingQuality () [0x00000] in :0

I don't quite understand why this is causing an error, all it's doing is returning a predefined int of 100. No errors or warnings show in the Editor at all.

Running on Xcode 9 (beta) - so it might be an internal issue with Xcode? Either way, would like to get the bottom of this, incase it cases me issues further down the line.

(Post; my formatting bar isn't loading in the web browser for some reason)