DiscordClient::Dispose() throws NullReferenceException if no succesfull connection was established

Created by: schaap

Summary

A NullReferenceException is thrown when a DiscordClient is destroyed before a succesfull conncection was established. The cause is _webSocketClient being dereferenced without checking whether it is null.

Steps to reproduce

1. Run the following code:

// No, do not change the token. It's invalid on purpose.
DiscordClient? client = new DiscordClient(new DiscordConfiguration{Token = "NotValid"});
try {
    await client.ConnectAsync();
} catch (Exception) {}
client.Dispose();
client = null;

Details

Error was enountered in the v4.0 nightly build NuGet packages (683) and the bug has been verified against the current source on Github.

Notes

Use of the C# 8.0 nullable feature might help in finding similar errors.