In your case, I think you need to use the default locator of Stage surface instead of null object.
For each surfaces in your stage, you'll find an empty list (1). right click on it to create locator points on the 3D surface. For some basic surface suich as plane or boxes, you can create them automatically thanks the "Create Default locators" option accessible by right clicking of the surface.
Then, in the mapper element of your video projection, right click on it (2) and choose the option "Create visible surface locator". then, use those points to warp your mapping in 2D
Hope it help!