if(ext->lua_actions[LV_BTN_ACTION_PR]) {          /*"my_press_action" in the example*/
       printf("Button address in C: 0x%xn", (int)btn);
       lua_State *L = lv_lua_get_context();
       lua_getglobal(L, "lv_lua_actions");
       lua_getfield(L, -1, ext->lua_actions[LV_BTN_ACTION_PR]);
       if (!lua_isnil(L, -1)) {                                 /* Call the function if it is registered with this name*/
           lua_pushlightuserdata(L, btn);                       /* POSSIBLY WRONG!!! push the argument */
           lua_pcall(L, 1, 1, 0);
           if (lua_isnumber(L, -1)) {                           /* Read result*/
               int lua_res = lua_tonumber(L, -1);
               if(lua_res == LV_RES_OK ||  lua_res == LV_RES_INV) res = lua_res;        /*Save result*/
               lua_pop(L, 1);                                   /* pop returned value */
           }
       }
   }
}

lv_lua_actions = {}         -- stores the action function

-- Register a Lua function for LittlevGL actions
local function register_handler(key, fn)
  lv_lua_actions[key] = fn
end

-- A test Press action
local function action_press(btn_arg)
  print("Pressed")
  print("btn_global:" .. tostring(btn_global))
  print("btn_arg:" .. tostring(btn_arg))
  lv.obj_set_width(btn_arg,  50)
  return lv.RES_OK
end

--Register a function
register_handler("my_press_action", action_press)

print("Create a button")
btn_global = lv.btn_create(lv.scr_act(), nil)
print("Button created: btn_global: "  .. tostring(btn_global))
lv.btn_set_lua_action(btn_global, lv.BTN_ACTION_PR, "my_press_action")

Create a button
Button created: btn_global: <lv_obj_t userdata: 0xb436a8>
...here I press the button...
Button address in C: 0x861c48
btn_global:<lv_obj_t userdata: 0xb436a8>
btn_arg:userdata: 0x861c48
...and the program crashes and exits...