Error extending classes

Sendbird Chat API/SDK JavaScript
1

[Problem/Question]
I want to extend Sendbird classes to add functions that will be used for various pages. For example, on GroupChannel class, I want to add a getChatmate button to get other user in the 1-to-1 class, plus other functions that will be helpful across our pages. However, when I do, I get the following error: TypeError: Cannot read properties of undefined (reading "channel_url")

I also saw this issue that is probably related, but can’t be sure that the cause of my issue is same as the answers there.

// If problem, please fill out the below. If question, please delete.
[SDK Version]
@sendbird/chat@4.6.1

[Reproduction Steps]
Sample code:

import {
  GroupChannel,
} from '@sendbird/chat/groupChannel'

export default class CustomGroupChannel extends GroupChannel {
  constructor(data) {
    super(data)
    Object.assign(this, data)
  }

  getChatmate (currentUser) {
    // do something
  }
}

[Frequency]
Always

[Current impact]
Cannot say at the moment