fix `CreatePostError` (#2546)

src/Discord.Net.Rest/Entities/Channels/ThreadHelper.cs

@@ -148,7 +148,7 @@ namespace Discord.Rest
                 throw new ArgumentException("The only valid MessageFlags are SuppressEmbeds and none.", nameof(flags));
 
             if (channel.Flags.HasFlag(ChannelFlags.RequireTag))
-                throw new ArgumentException($"The channel {channel.Name} requires posts to have at least one tag.");
+                Preconditions.AtLeast(tagIds?.Length ?? 0, 1, nameof(tagIds), $"The channel {channel.Name} requires posts to have at least one tag.");
 
             var args = new CreatePostParams()
             {