Bug fix:

When 2nd window open, the code that tried to find an Image editor in the
the active window failed.

source/blender/editors/render/render_internal.c

@@ -347,6 +347,8 @@ static ScrArea *find_area_showing_r_result(bContext *C)
 					break;
 			}
 		}
+		if(sa)
+			break;
 	}
 
 	return sa;