Give private users a proper ID.

src/Discord.Net/DiscordClient.Members.cs

@@ -10,7 +10,7 @@ namespace Discord
 		public Members(DiscordClient client, object writerLock)
 			: base(client, writerLock, x => x.OnCached(), x => x.OnUncached()) { }
 		private string GetKey(string userId, string serverId)
-			=> serverId + '_' + userId;
+			=> (serverId ?? "Private") + '_' + userId;
 
 		public User this[string userId, string serverId]
 			=> this[GetKey(userId, serverId)];

src/Discord.Net/DiscordClient.cs

@@ -549,7 +549,7 @@ namespace Discord
 							var channel = _channels[data.ChannelId];
 							if (channel != null)
 							{
-								var user = _members[data.UserId, channel.Server.Id];
+								var user = _members[data.UserId, channel.Server?.Id];
 
 								if (user != null)
 								{