public class MyBaseClass
{
    public DataSet myDataSet;
    
    public void MyMethod( string cStr )
    {
          // do something with the string cStr
   }
}

public class MyDerivedClass : MyBaseClass
{
               // Nothing is this class, so far
 }

public partial class frmMyForm : Form
{
   private string cStr
   public MyDefirvedClass m_DerivedClass;

   public frmMyForm()
   {
         InitializeComponent();
         m_DerivedClass.MyMethod( cStr );                        //  !!! This where I get error Null Reference
   }

}