Fix KeyNotFoundException for stickers in messages

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -511,7 +511,12 @@ namespace Discord.WebSocket
             if (model.GuildId.IsSpecified)
             {
                 var guild = State.GetGuild(model.GuildId.Value);
-                sticker = guild.AddOrUpdateSticker(model);
+
+                // since the sticker can be from another guild, check if we are in the guild or its in the cache
+                if (guild != null)
+                    sticker = guild.AddOrUpdateSticker(model);
+                else
+                    sticker = SocketSticker.Create(this, model);
                 return sticker;
             }
             else

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

@@ -1240,10 +1240,10 @@ namespace Discord.WebSocket
         /// </returns>
         public async ValueTask<SocketCustomSticker> GetStickerAsync(ulong id, CacheMode mode = CacheMode.AllowDownload, RequestOptions options = null)
         {
-            var sticker = _stickers[id];
+            var sticker = _stickers.FirstOrDefault(x => x.Key == id);
 
-            if (sticker != null)
-                return sticker;
+            if (sticker.Value != null)
+                return sticker.Value;
 
             if (mode == CacheMode.CacheOnly)
                 return null;

src/Discord.Net.WebSocket/Entities/Messages/SocketUserMessage.cs

@@ -174,17 +174,16 @@ namespace Discord.WebSocket
                         SocketSticker sticker = null;
 
                         if (guild != null)
-                        {
                             sticker = guild.GetSticker(stickerItem.Id);
-                        }
 
-                        if(sticker == null)
-                        {
+                        if (sticker == null)
                             sticker = Discord.GetSticker(stickerItem.Id);
-                        }
 
                         // if its still null, create an unknown
-                        sticker = SocketUnknownSticker.Create(Discord, stickerItem);
+                        if (sticker == null)
+                            sticker = SocketUnknownSticker.Create(Discord, stickerItem);
+
+                        stickers.Add(sticker);
                     }
 
                     _stickers = stickers.ToImmutable();

src/Discord.Net.WebSocket/Entities/Stickers/SocketCustomSticker.cs

@@ -70,7 +70,7 @@ namespace Discord.WebSocket
 
         internal SocketCustomSticker Clone() => MemberwiseClone() as SocketCustomSticker;
 
-        private string DebuggerDisplay => $"{Name} in {Guild.Name} ({Id})";
+        private new string DebuggerDisplay => Guild == null ? base.DebuggerDisplay : $"{Name} in {Guild.Name} ({Id})";
 
         // ICustomSticker
         ulong? ICustomSticker.AuthorId

src/Discord.Net.WebSocket/Entities/Stickers/SocketSticker.cs

@@ -76,7 +76,7 @@ namespace Discord.WebSocket
             }
         }
 
-        private string DebuggerDisplay => $"{Name} ({Id})";
+        internal string DebuggerDisplay => $"{Name} ({Id})";
 
         /// <inheritdoc/>
         public override bool Equals(object obj)