NullReferenceException at MySql.Data.MySqlClient.MySqlCommand.EndExecuteNonQueryNot An Issue
MySql handle raised an exception (NullReferenceException: Object reference not set to an instance of an object)
at MySql.Data.MySqlClient.MySqlCommand.EndExecuteNonQuery (System.IAsyncResult asyncResult) [0x00022] in <8d7380b3d3cf4bd8be324c584f371b44>:0

at (wrapper remoting-invoke-with-check) MySql.Data.MySqlClient.MySqlCommand.EndExecuteNonQuery(System.IAsyncResult)
at Oxide.Core.MySql.Libraries.MySql+MySqlQuery.Handle () [0x000f2] in <7a75fa05cbc04391919cf072f240bf30>:0

When player connecting

This may indicate something that should exist in your MySQL database table doesn't exist.
Re-create the DB and reload plugin should solve it

I tried that. Still after people joined:

MySql handle raised an exception (NullReferenceException: Object reference not set to an instance of an object)
at MySql.Data.MySqlClient.MySqlCommand.EndExecuteNonQuery (System.IAsyncResult asyncResult) [0x00022] in <8d7380b3d3cf4bd8be324c584f371b44>:0

at (wrapper remoting-invoke-with-check) MySql.Data.MySqlClient.MySqlCommand.EndExecuteNonQuery(System.IAsyncResult)
at Oxide.Core.MySql.Libraries.MySql+MySqlQuery.Handle () [0x000f2] in <7a75fa05cbc04391919cf072f240bf30>:0
Locked automatically