Hey again,
is there any error that is thrown in the very beginning when you enable the plugin before `Nonexistent function 'process' in base 'Nil'.`? I suspect there is a problem with creating the PlacementController object. Can you open the files "res://addons/assetplacer/viewport_3d_controllers/placement_controller/placement_controller.gd" and "res://addons/assetplacer/viewport_3d_controllers/viewport_3d_controller.gd" inside the script editor and see if any compile errors are thrown? Maybe a class you are registering collides with my controller classes.
In any case, try if adding the AssetPlacer to a clean project works. If you prefer, you could also send me your project and I'll see if I can find the cause of conflict.