Error extending classes

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.

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[SDK Version]

[Reproduction Steps]
Sample code:

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

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

  getChatmate (currentUser) {
    // do something


[Current impact]
Cannot say at the moment