Fix cache errors on server delete

src/Discord.Net/Collections/Channels.cs

@@ -28,16 +28,23 @@ namespace Discord.Collections
 		protected override void OnRemoved(Channel item)
 		{
 			if (!item.IsPrivate)
+			{
+				var server = item.Server;
+				if (server != null)
 					item.Server.RemoveChannel(item.Id);
+			}
 			if (item.RecipientId != null)
 			{
 				var user = item.Recipient;
+				if (user != null)
+				{
 					if (user.PrivateChannelId != item.Id)
 						throw new Exception("User has a different private channel.");
 					user.PrivateChannelId = null;
 					user.RemoveRef();
 				}
 			}
+        }
 
 		internal Channel this[string id] => Get(id);
 		

src/Discord.Net/Collections/Members.cs

@@ -24,11 +24,19 @@ namespace Discord.Collections
 		}
 		protected override void OnRemoved(Member item)
 		{
-			item.Server.RemoveMember(item);
+			var server = item.Server;
-			item.User.RemoveServer(item.ServerId);
+			if (server != null)
-			item.User.RemoveRef();
+			{
+				server.RemoveMember(item);
 				if (item.UserId == _client.CurrentUserId)
-				item.Server.CurrentMember = null;
+					server.CurrentMember = null;
+			}
+			var user = item.User;
+			if (user != null)
+			{
+				user.RemoveServer(item.ServerId);
+				user.RemoveRef();
+			}
 		}
 		
 		internal Member this[string userId, string serverId]

src/Discord.Net/Collections/Messages.cs

@@ -22,8 +22,12 @@ namespace Discord.Collections
 		}
 		protected override void OnRemoved(Message item)
 		{
-			item.Channel.RemoveMessage(item.Id);
+			var channel = item.Channel;
-			item.User.RemoveRef();
+			if (channel != null)
+				channel.RemoveMessage(item.Id);
+			var user = item.User;
+			if (user != null)
+				user.RemoveRef();
 		}
 
 		internal Message this[string id] => Get(id);

src/Discord.Net/Collections/Roles.cs

@@ -18,6 +18,8 @@ namespace Discord.Collections
 		}
 		protected override void OnRemoved(Role item)
 		{
+			var server = item.Server;
+			if (server != null)
 				item.Server.RemoveRole(item.Id);
 		}