Fixed nullrefs when message cache is disabled

src/Discord.Net.WebSocket/Entities/Channels/SocketDMChannel.cs

@@ -82,9 +82,9 @@ namespace Discord.WebSocket
             => ChannelHelper.EnterTypingState(this, Discord, options);
 
         internal void AddMessage(SocketMessage msg)
-            => _messages.Add(msg);
+            => _messages?.Add(msg);
         internal SocketMessage RemoveMessage(ulong id)
-            => _messages.Remove(id);
+            => _messages?.Remove(id);
 
         //Users
         public new SocketUser GetUser(ulong id)

src/Discord.Net.WebSocket/Entities/Channels/SocketGroupChannel.cs

@@ -105,9 +105,9 @@ namespace Discord.WebSocket
             => ChannelHelper.EnterTypingState(this, Discord, options);
 
         internal void AddMessage(SocketMessage msg)
-            => _messages.Add(msg);
+            => _messages?.Add(msg);
         internal SocketMessage RemoveMessage(ulong id)
-            => _messages.Remove(id);
+            => _messages?.Remove(id);
 
         //Users
         public new SocketGroupUser GetUser(ulong id)