Why do I get 'a null value was found where an object instance was required' pointing to an empty function?

My game works totally fine in editor but when I try to build to iOS Xcode gives me this error pointing to SimpleDrag.Start() The game loads but goes completely black straight after the load screen although continues to run (I think). The Start() function in SImpleDrag.cs script has no code inside it. What can I do? I am using Xcode 7.3.1 and Unity 5.3.5 Cheers