DataGridViewError: Conversion from type DBNull to type String is not valid

Dear Sir,

I have an error "Conversion from type 'DBNull' to type 'String' is not valid " when creating an empty in the  column Quantity, Price, Discount and I've used handle the error in DataGridView DataError event handler but still an error 

In my code, I have marked which code is an error "'error in below line code"

 can I resolve the error with 2 alternatives:

1. First, keep the datagridview column empty

2. The second automatically returns the previous first value

Please Guide me

Thanks

Public Class Form1
    Public _myTable As New DataTable()
    Protected Overrides Sub OnLoad(e As EventArgs)
        MyBase.OnLoad(e)
        _myTable.Columns.AddRange({
    New DataColumn("Code", GetType(String)),
    New DataColumn("Quantity", GetType(Integer)),
    New DataColumn("Price", GetType(Integer)),
    New DataColumn("Discount", GetType(Integer)),
     New DataColumn("Total", GetType(Integer))})
    End Sub
    Private Sub CalculateTotalqty()
        Dim tot As Double = 0
        For Each item As DataGridViewRow In DataGridView1.Rows
            Dim val As Double
            'error in below line code
            Double.TryParse(CType(item.Cells("Quantity").Value, String), val)
            tot += val
        Next item
        lblQty.Text = tot.ToString("N2")
        'lblChange.Text = (cash - tot).ToString("N2")
    End Sub
    Private Sub CalculateTotalValue()
        Dim tot As Double = 0
        For Each item As DataGridViewRow In DataGridView1.Rows
            Dim val As Double
            Dim val2 As Double
            Dim val3 As Double
            Double.TryParse(CType(item.Cells("Quantity").Value, String), val)
            'error in below line code
            Double.TryParse(CType(item.Cells("Price").Value, String), val2)
            'error in below line code
            Double.TryParse(CType(item.Cells("Discount").Value, String), val3)
            tot += (val * val2) - (val3)
        Next item
        lblValue.Text = tot.ToString("N")
        'lblChange.Text = (cash - tot).ToString("N2")
    End Sub
    Private Sub BtnAddproduct_Click(sender As Object, e As EventArgs) Handles BtnAddproduct.Click
        _myTable.Rows.Add("A", 2, 10000, 0, 20000)
        DataGridView1.AutoGenerateColumns = False
        ' Disable adding new row
        DataGridView1.AllowUserToAddRows = False
        ' Create our medium between grid and collection
        DataGridView1.DataSource = _myTable
        CalculateTotalqty()
        CalculateTotalValue()
    End Sub
    Private Sub grid_CellEndEdit(sender As Object, e As DataGridViewCellEventArgs) Handles DataGridView1.CellEndEdit
        CalculateTotalqty()
        CalculateTotalValue()
        Dim price As Integer = Nothing, quantity As Integer = Nothing, Discount As Integer = Nothing, total As Integer = Nothing
        If DataGridView1.Rows(e.RowIndex).Cells("Quantity").Value IsNot Nothing Then
            If Not Integer.TryParse(DataGridView1.Rows(e.RowIndex).Cells("Quantity").Value.ToString(), quantity) Then
                quantity = 0
            End If
        Else
            quantity = 0
        End If
        price = Integer.Parse(DataGridView1.Rows(e.RowIndex).Cells("Price").Value.ToString())
        Discount = Integer.Parse(DataGridView1.Rows(e.RowIndex).Cells("Discount").Value.ToString())
        total = (quantity * price) - (Discount)
        DataGridView1.Rows(e.RowIndex).Cells("Total").Value = total
    End Sub
    Private Sub DataGridView1_DataError(sender As Object, e As DataGridViewDataErrorEventArgs) Handles DataGridView1.DataError
        If e.Context = DataGridViewDataErrorContexts.Commit Then
            MessageBox.Show(String.Format("Column {0} value is required.", e.ColumnIndex))
        End If
    End Sub
End Class