Reproduction steps:
1. Download user's project and open in Unity
2. Open "Emissive" scene
3. Enter Play mode

Expected: OnRenderImage method's source should not be null after entering the method the second time (while debugging, first time hitting OnRenderImage has a good source)
Actual: source is null after entering a second time and error appears in the console

Reproduced on: 5.6.5p2; 2017.1.3p2; 2017.2.2p1; 2017.3.1p4; 2018.1.0b11; 2018.2.0a3

Note: error message
NullReferenceException: Object reference not set to an instance of an object
UnityEngine.PostProcessing.RenderTextureFactory.Get (UnityEngine.RenderTexture baseRenderTexture) (at Assets/PostProcessing/Runtime/Utils/RenderTextureFactory.cs:19)
UnityEngine.PostProcessing.PostProcessingBehaviour.OnRenderImage (UnityEngine.RenderTexture source, UnityEngine.RenderTexture destination) (at Assets/PostProcessing/Runtime/PostProcessingBehaviour.cs:259)