System.NullReferenceException: Object reference not set to an instance of an object.
This simply means that you are trying to access a property of an object that is null.
Using Visual Studio DEBUGGER will tell you exact line and the object that is null and thus throwing an error.
For now, it looks like there is an issue here:
ConnString = ConfigurationManager.ConnectionStrings["AMS - Database"].ConnectionString;
And thus, when you try to use it and open a connection in this line, it throws an error.
using (SqlConnection con = new SqlConnection(ConnString1))
Most probably your connection string is not initialized properly.