IndexOutOfRange Exception When Click On DataGridView Cell

Question

Last Week I'm Working On A WinFroms Project And I Created A Cart Form. In Cart, There Is A DataGridView Control & It Contains A Column Named purchasedQuantity.
purchasedQuantity Is Editable & User Can Enter Value Only In This Column. Here Is The EvenHandler For CellEndEdit

private void dataGridView1_CellEndEdit(object sender, DataGridViewCellEventArgs e) 
{
    if (e.RowIndex <= 0 || e.ColumnIndex != 4)
        return;

    int row = e.RowIndex;
    int demandedQuantity = int.Parse(this.dataGridView1.Rows[row].Cells["purchasedQuantity"].Value.ToString());
    int inStockQunatity = int.Parse(this.dataGridView1.Rows[row].Cells["unitsInStock"].Value.ToString());
    double price = double.Parse(this.dataGridView1.Rows[row].Cells["unitPrice"].Value.ToString());

    if (demandedQuantity <= inStockQunatity)
    {
        this.dataGridView1.Rows[row].Cells["subTotal"].Value = (demandedQuantity * price).ToString();
        this.dataGridView1.Rows[row].Cells["unitsInStock"].Value = (inStockQunatity - demandedQuantity).ToString();
        this.dataGridView1.Rows[row].Cells["purchasedQuantity"].Style.BackColor = System.Drawing.Color.White;
    }
    else
    {
        MessageBox.Show(string.Format("We Have Only {0} Left In The Stock..!!", inStockQunatity.ToString()), "Error", MessageBoxButtons.OK);
        this.dataGridView1.Rows[row].Cells["purchasedQuantity"].Style.BackColor = System.Drawing.Color.Red;
    }
}

The Problem Is That Whenever I Click On Any Cell Of The DataGridView, I Get The Following Error:

An unhandled exception of type 'System.IndexOutOfRangeException' occurred in System.Windows.Forms.dll Additional information: Index -1 does not have a value.
Make sure that the maximum index on a list is less than the list size
Make sure the index is not a negative number.
Make sure data column names are correct.

I Have A Home Form That Create ViewProduct Form Instance And That ViewProduct Form Creates Cart Form Instance & I Get Error In The Home Form In The Following Function:
Home => ViewProdcuts => Cart

 private void ShowForm(Form form) 
 {
      this.Hide();
      form.ShowDialog();

      form.Close();
      this.Show();
 }

On The Following Line:

form.ShowDialog();

As I Mentioned First, I Start Working On It Last Week. At That Time It Works Fine, No Exceptions Are Thrown On Clicking On Any Row & I Was Able To Enter Value In The purchasedQuanttiy Cell.
After One Week It Get Angry, Don't Know Why (Just Kidding)
I Googled That Problem But Found No Answer. Can Anybody Please Tell Me What Is Wrong With This Code.
Thanks In Advance For Reply