Null value upon connecting to multiplayer.

[Houyami]

[21:11:10] [Main Thread/WARN] [tML]: Silently Caught Exception: System.ArgumentNullException: Value cannot be null. (Parameter 'key') at System.Collections.Generic.Dictionary2.FindValue(TKey key) at System.Collections.Generic.Dictionary2.ContainsKey(TKey key) at SubworldLibrary.SubworldLibrary.SendToSubservers(ISocket socket, Byte[] data, Int32 start, Int32 length, Object& state) in SubworldLibrary.cs:line 682 at DMDTerraria.Net.Sockets.TcpSocket::Terraria.Net.Sockets.ISocket.AsyncSend(TcpSocket this, Byte[] data, Int32 offset, Int32 size, SocketSendCallback callback, Object state) at Terraria.Net.NetManager.SendData(ISocket socket, NetPacket packet) in tModLoader\Terraria\Net\NetManager.cs:line 93 at Terraria.GameContent.Bestiary.NPCWasChatWithTracker.OnPlayerJoining(Int32 playerIndex) in tModLoader\Terraria\GameContent\Bestiary\NPCWasChatWithTracker.cs:line 75 at Terraria.GameContent.Bestiary.BestiaryUnlocksTracker.OnPlayerJoining(Int32 playerIndex) in tModLoader\Terraria\GameContent\Bestiary\BestiaryUnlocksTracker.cs:line 38 at DMDTerraria.MessageBuffer::GetData(MessageBuffer this, Int32 start, Int32 length, Int32& messageType) at DMDTerraria.NetMessage::CheckBytes(Int32 bufferIndex) at Terraria.Netplay.UpdateServerInMainThread() in tModLoader\Terraria\Netplay.cs:line 64 at Terraria.Main.DoUpdate(GameTime& gameTime) in tModLoader\Terraria\Main.cs:line 12729 at Terraria.Main.Update(GameTime gameTime) in tModLoader\Terraria\Main.cs:line 12618 at DMDTerraria.Main::DedServ_PostModLoad(Main this, Boolean& reloadMods) at Terraria.Main.DedServ() in tModLoader\Terraria\Main.cs:line 4144 at Terraria.Program.LaunchGame_(Boolean isServer) in tModLoader\Terraria\Program.cs:line 231 at Terraria.Program.LaunchGame(String[] args, Boolean monoArgs) in tModLoader\Terraria\Program.cs:line 194 at MonoLaunch.<>c__DisplayClass2_0.<Main>b__0() in tModLoader\Terraria\MonoLaunch.cs:line 65 at System.Threading.Thread.StartCallback()