If you’re widget is off screen you have to have “Tick when offscreen” enable/set to true.
They by default disable rendering it if it isn’t going to be seen.
But in order to update the render target it needs to be ticking.
So if you make it invisible by setting its visibility flag to false or have it offscreenn without the “Tick when offscreen” enable then you will get null for “Get Render Target”.