In forward rendering in 5.4.x, in the first call of OnRenderImage, source is null (results in null ref exception) when effects are enabled in scene view.

Repro:

1) Open attached project and scene
2) View nullref in console (playmode results in a new nullref)

Optional check:

3) Disable scene view effects (playmode now results in no nullref)
4) Add "if (source == null){return;}" to "OnRenderImage" method (playmode now results in no nullref)

Notes:
* Only first call returns null (on recompile in edit or first frame of play)
* Requires effects enabled in scene view (so requires 5.4)
* Doesnt repro in deferred shading
* Bloom (used in repro project) will now work if repulled from trunk (fixed by null check PR)