Hello,

Maybe there was an error before it.

So the "discussCoreCommonService" object was not loaded.

If it is the first error. You can try to add the line:

import { DiscussCoreCommon} from "../common/discuss_core_common_service";

before the line:

export class ChannelSelector extends Component {

in the file:

odoo/addons/mail/static/src/discuss/core/web/channel_selector.js

Then refresh your browser cache to make it load the updated .js file.

Or enable odoo debug mode with assets. 

Stanislav