Trying to apply a user settings fails with a 400 bad request error

[melslow]

When using /gcal settings and clicking one of the options nothing happens. When looking into the Dev Console you can see the following request being responded to with a error 400.

When inspecting the request you can see that the selected_option is empty.

POST https://mattermost.xxxx.com/api/v4/posts/~redacted~/actions/~redacted~

{"selected_option":""}

The server then responds with:

{
    "id": "api.post.do_action.action_integration.app_error",
    "message": "Action integration error.",
    "detailed_error": "",
    "request_id": "<redacted>",
    "status_code": 400
}

Server version is 8.1.6 Plugin version is 1.0.1

How to reproduce

1. Setup the plugin
2. Go through gcal connect
3. execute gcal settings
4. Click any of the settings
5. Nothing happens. Bad request error in the dev console

Could someone point me to where the code for this functionality lives? I tried looking into it however its not clear to me which part of the code handles this.