Server.log catch of what causes the crash when you are hosting and someone joins the server.

[00:16:20.623] [Server Loop Thread/ERROR] [tML]: Unhandled Exception
System.NullReferenceException: Object reference not set to an instance of an object.
at DragonLens.Content.Tools.Multiplayer.PlayerManagerUpdater.PlayerDisconnect() in DragonLens\Content\Tools\Multiplayer\PlayerManager.cs:line 54
at Terraria.ModLoader.PlayerLoader.PlayerDisconnect(Int32 playerIndex) in tModLoader\Terraria\ModLoader\PlayerLoader.cs:line 1257
at Terraria.Player.Hooks.PlayerDisconnect(Int32 playerIndex) in tModLoader\Terraria\Player.cs:line 269
at Terraria.NetMessage.SyncOnePlayer(Int32 plr, Int32 toWho, Int32 fromWho) in tModLoader\Terraria\NetMessage.cs:line 2740
at DMD<System.Void Terraria.Netplay:UpdateConnectedClients()>()
at SyncProxy<System.Void Terraria.Netplay:UpdateConnectedClients()>()
at Terraria.Netplay.ServerLoop()
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)