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On line 6 in the update function you are not checking if tiles still exist which after fliping the table it will get deleted.. you are also checking the tiles before checking if exit_tile_added is true. Its been a while since I've done lua coding but I would guess the tiles.getQuantity() check happens before the exit_tile_added check, meaning that during the end game you are wasting time checking tiles.getQuantity() when exit_tile_added is already true.
Updating line 6 to
if exit_tile_added == false and tiles ~= null and tiles.getQuantity() < 6 then
Should fix both problems though I've not actually tested the code. So you are first checking if the exit tile flag exists, if tiles is not null and then checking getQuantity, so the check will fail before getQuantity is called if the exit tile flag is set or if the game couldn't find the tiles varible.