How to blit a rendertexture to screen(GameView in Editor)?

I use the Graphic.Blit with null target parameter like this:

 void Update () 
     {
         rt.DiscardContents ();
 
         RenderTexture curRT = RenderTexture.active;
         RenderTexture.active = rt;
 
         mainCam.targetTexture = rt;
         mainCam.Render ();
         mainCam.targetTexture = null;
 
         RenderTexture.active = curRT;
         Graphics.Blit (rt, null as RenderTexture);
     }

In Built Player it's working, but in Editor the display1 of GameView says "No Cameras Rendering":

and the Camera is set like this:

I need to see and debug in the Editor without building a player, so what possibly could be the problem?

my platform is WIn7 64bit and Unity Pro 5.4.0 b20

Thanks in advance.