My bad!
At the bottom of the sprite struct I do this:
GML:
sprite_list_master_add(weak_ref_create(self));
and this part works, it does add itself to the list.
The error I get is:
############################################################################################
ERROR in
action number 1
of Step Event0
for object obj_manager_game:
Variable <unknown_object>.step(100028, -2147483648) not set before reading it.
at gml_Script_sprite_list_master_step (line 77) - step();
############################################################################################
gml_Script_sprite_list_master_step (line 77)
gml_Object_obj_manager_game_Step_0 (line 3) - sprite_list_master_step();
It seems like even though it returns the sprite struct it isn't treating it as the sprite struct, so it can't find the step function I put in it. I don't know if I found a bug or if I'm not using this correctly.