When I insert in database through connectionless i found a error null exception

Question

Here I found error in newrow

 DataRow drnew = ds.Tables["SMaster"].NewRow();// here i found error 
 drnew[0] = Convert.ToInt32(textBox1.Text);
 drnew[1] = textBox2.Text;`enter code here`
 drnew[2] = textBox3.Text;
 drnew[3] = Convert.ToInt32(textBox4.Text);
 ds.Tables["SMaster"].Rows.Add(drnew);
 da.Update(ds, "Smaster");
 MessageBox.Show("Record added");
 dataGridView1.DataSource = ds.Tables["Smaster"];