GetGatewayInfoAsync throws NullReferenceException when an invalid token has been provided to DiscordShardedClient

Created by: Soyvolon

Summary

Providing an invalid token to a DiscordShardedClient and calling UseCommandsNextAsync will call GetGatewayInfoAsync, but if there is an invalid token the gateway response, info will be populate with null values that are never checked.

Details

.NET Core 3.1 D#+ Nightly-00725 (Latest from master branch as of 9/23/2020 10:00 EST)

Using an invalid token will cause var resp = await http.GetAsync(url).ConfigureAwait(false); to get a response of 401 Unauthorized, but there is no check in GetGatewayInfoAsync to see if the response was valid or not. When it is parsed to the GatewayInfo object, all values are either null or 0, causing the NullReferenceException.

        private async Task<GatewayInfo> GetGatewayInfoAsync()
        {
            // . . .
            var info = jo.ToObject<GatewayInfo>();

            //There is a delay from parsing here.
            timer.Stop();
            // info.SessionBucket is null if there is an invalid token.
            info.SessionBucket.resetAfter -= (int)timer.ElapsedMilliseconds; 
            // . . .
        }

Steps to reproduce

1. Use DiscordShardedClient with an invalid token.
2. Call DiscordShardedClient.GetCommandsNextAsync before using StartAsync.

Notes

When using a DiscordClient with an invalid token, you get this exception:

throw new Exception("Authentication failed. Check your token and try again.", e);

Throwing a similar exception from GetGatewayInfoAsync when the response from Discord is 401 Unauthorized will help users know what the problem is.