Add new box that contain a fullbox

[TimYao18]

Hi, I want to add a new box 'grpl' and I write it as below.
I added it into init.py, but I don't know what's wrong that it ran and stop.
Can you tell me how to fix this?

# -*- coding: utf-8 -*-
from .box import Box
from .box import FullBox
from .box import Quantity
from .box import read_uint
from .box import read_fixed_size_string
from .box import read_utf8string


# ISO/IEC 14496-12:2022, Section 8.18.2
class GroupListBox(Box):
    box_type = b"grpl"
    is_mandatory = False
    quantity = Quantity.ZERO_OR_ONE

    def read(self, file):
        self.ster_box = self.read_box(file)
        self.group_id = read_uint(file, 4)
        self.num_entities_in_group = read_uint(file, 4)
        self.entity_ids = []
        for _ in range(self.num_entities_in_group):
             self.entity_ids.append(read_uint(file, 4))

    def contents(self):
        tuples = super().contents()
        tuples += (("entity_group", self.ster_box.contents()),)
        tuples += (("group_id", self.group_id),)
        tuples += (("num_entities_in_group", self.num_entities_in_group),)
        for id in enumerate(self.entity_ids):
            tuples += (("entity_id", id),)
        return tuples

# ISO/IEC 23008-12:2022, Section 6.8.5
class StereoPairBox(FullBox):
    box_type = b"ster"
    is_mandatory = True
    quantity = Quantity.EXACTLY_ONE

[chemag]

You need to provide an example of how this breaks. Make sure you add the binary file that you're trying to parse