How to handle invalid cast operation

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C#

WinForms

I have bankname combobox,accountno textbox and Accounttype combobox.If i select bankname from the combobox.It shows the related data in datagridview from database table.If select
"Select " text from the bankcombobox.It doesn't show any data in datagridview.I want to add new row top of the datagridview when i click addbutton.new row is adding succesfully.In datagridview i have textboxes like slno,startno,noofcheques and endingno and status.after entering startno and noofcheques both will add populate data in ending no text for that i am using cell leave event.when i load the form firsttime new row is adding and values are calculating fine.when i first select bankname and then select "select" text click addnew row button row is adding but when i try to enter values into the startno textbox.It throws error.'System.InvalidCastException' occurred in mscorlib.dll.Object cannot be cast from DBNull to other types.
I'm fresher please help me on this.

What I have tried:

       private void dgvwChqs_CellLeave(object sender, DataGridViewCellEventArgs e)
       {

          if (dgvwChqs.Rows[e.RowIndex].Cells["startno"].Value != null)
           {
               dgvwChqs.Rows[e.RowIndex].Cells["endingno"].Value =
           Convert.ToInt32(dgvwChqs.Rows[e.RowIndex].Cells["startno"].Value) + Convert.ToInt32(dgvwChqs.Rows[e.RowIndex].Cells["noofcheques"].Value);
           }
       }


private void btnadd_Click(object sender, EventArgs e)
       {

           dts.Rows.InsertAt(dts.NewRow(), 0);



           }