Fix messages in DMs

Myriad/Cache/DiscordCacheExtensions.cs

@@ -1,8 +1,6 @@
 using System.Threading.Tasks;
 
 using Myriad.Gateway;
-using Myriad.Rest;
-using Myriad.Types;
 
 namespace Myriad.Cache
 {
@@ -51,6 +49,11 @@ namespace Myriad.Cache
 
         private static async ValueTask SaveMessageCreate(this IDiscordCache cache, MessageCreateEvent evt)
         {
+            // DM messages don't get Channel Create events first, so we need to save
+            // some kind of stub channel object until we get the real one
+            if (evt.GuildId == null)
+                await cache.SaveDmChannelStub(evt.ChannelId);
+            
             await cache.SaveUser(evt.Author);
             foreach (var mention in evt.Mentions) 
                 await cache.SaveUser(mention);

Myriad/Cache/IDiscordCache.cs

@@ -11,6 +11,7 @@ namespace Myriad.Cache
         public ValueTask SaveChannel(Channel channel);
         public ValueTask SaveUser(User user);
         public ValueTask SaveRole(ulong guildId, Role role);
+        public ValueTask SaveDmChannelStub(ulong channelId);
 
         public ValueTask RemoveGuild(ulong guildId);
         public ValueTask RemoveChannel(ulong channelId);

Myriad/Cache/MemoryDiscordCache.cs

@@ -81,6 +81,18 @@ namespace Myriad.Cache
             return default;
         }
 
+        public ValueTask SaveDmChannelStub(ulong channelId)
+        {
+            // Use existing channel object if present, otherwise add a stub
+            // We may get a message create before channel create and we want to have it saved
+            _channels.GetOrAdd(channelId, id => new Channel
+            {
+                Id = id, 
+                Type = Channel.ChannelType.Dm
+            });
+            return default;
+        }
+
         public ValueTask RemoveGuild(ulong guildId)
         {
             _guilds.TryRemove(guildId, out _);