By default the render node is added to the main scene, so specifying that in your code is redundant. Your graphics card might not support render textures of size [1024,1024]. Try adding the following line after you create the render node to see if it helps:
Code:
lens.setBuffer(viz.RENDER_FRAME_BUFFER)
This will use the standard frame buffer to copy to the render texture, instead of FBOs. Also, make sure your graphics card driver is up to date.