post process shaders break after changing anything

I can't seem to find any documentation anywhere on post processing with shader forge so this may be an unintended use of SF, but trying to recreate how I'd do post processes in Unreal. 

Start off simple, just throw in the camera's depth, all hunky dory.



now the problem comes as soon as I edit/compile that shader again, see the simple edit below


screen goes entirely black and there's nothing which can be done about it.
change all the settings, put them back the way they were, nothing,

The only work around seems to be to make a duplicate of the shader, rename it's reference in shader forge and then reapply it seems to work ... untill you want to edit it.

Any advice? I'll post the C# I'm using incase that's anything to do with it

using UnityEngine;
using System.Collections;

[ExecuteInEditMode]
[RequireComponent (typeof(Camera))]

public class MaskPost : MonoBehaviour {
}