This exception will obviously come when you'll try to point an object which is null.
Try this:
<connectionstrings>
<add name="ConnStringDb1" connectionstring="Data Source=localhost;Initial Catalog=YourDataBaseName;Integrated Security=True;" providername="System.Data.SqlClient" />
</connectionstrings>
To read the connection string into your code, use the ConfigurationSettings class.
if(ConfigurationManager.ConnectionStrings["myConnectionString"] != null){
string connStr = ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString;
}
--Amit