fix file playback

TomatenMusicCore/Music/Entitites/FullTrackContext.cs

@@ -20,7 +20,7 @@ namespace TomatenMusic.Music.Entitites
         public IEnumerable<string> YoutubeTags { get; set; }
         public ulong YoutubeViews { get; set; }
         public ulong YoutubeLikes { get; set; }
-        public Uri YoutubeThumbnail { get; set; }
+        public Uri? YoutubeThumbnail { get; set; } = null;
         public DateTime YoutubeUploadDate { get; set; }
         //
         // Summary:

TomatenMusicCore/Music/GuildPlayer.cs

@@ -109,7 +109,7 @@ namespace TomatenMusic.Music
             MusicActionResponse response;
             try
             {
-                response = PlayerQueue.(true);
+                response = PlayerQueue.NextTrack(true);
             }catch (Exception ex)
             {
                 if (Autoplay)

TomatenMusicCore/Util/Common.cs

@@ -32,10 +32,12 @@ namespace TomatenMusic.Util
             DiscordEmbedBuilder builder = new DiscordEmbedBuilder()
                 .WithTitle(track.Title)
                 .WithUrl(track.Source)
-                .WithImageUrl(context.YoutubeThumbnail)
                 .WithDescription(context.YoutubeDescription)
                 .AddField("Length", Common.GetTimestamp(track.Duration), true);
 
+            if (context.YoutubeThumbnail != null)
+                builder.WithImageUrl(context.YoutubeThumbnail);
+
             if (context.IsFile)
             {
                 builder.WithAuthor(track.Author);