fix: handle GUILD_DELETE behavior correctly (#1542)

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -1771,17 +1771,7 @@ namespace Discord.WebSocket
             return guild;
         }
         internal SocketGuild RemoveGuild(ulong id)
-        {
-            var guild = State.RemoveGuild(id);
-            if (guild != null)
-            {
-                foreach (var _ in guild.Channels)
-                    State.RemoveChannel(id);
-                foreach (var user in guild.Users)
-                    user.GlobalUser.RemoveRef(this);
-            }
-            return guild;
-        }
+            => State.RemoveGuild(id);
 
         /// <exception cref="InvalidOperationException">Unexpected channel type is created.</exception>
         internal ISocketPrivateChannel AddPrivateChannel(API.Channel model, ClientState state)

src/Discord.Net.WebSocket/Entities/Guilds/SocketGuild.cs

@@ -809,13 +809,14 @@ namespace Discord.WebSocket
             var members = Users;
             var self = CurrentUser;
             _members.Clear();
-            _members.TryAdd(self.Id, self);
+            if (self != null)
+                _members.TryAdd(self.Id, self);
 
             DownloadedMemberCount = _members.Count;
 
             foreach (var member in members)
             {
-                if (member.Id != self.Id)
+                if (member.Id != self?.Id)
                     member.GlobalUser.RemoveRef(Discord);
             }
         }