[2005] Problem with printing

[nader]

I tried this code to print from datagridview but it show me error at this line in line += cell.Value.ToString
the eroro
Object reference not set to an instance of an object.
How to fix this problem

Private Sub Button1_Click
 Dim PrintDoc As New PrintDocument
        AddHandler PrintDoc.PrintPage, AddressOf Me.PrintDGV
        PrintDoc.Print()
End Sub

Private Sub PrintDGV(ByVal sender As Object, _
  ByVal ev As PrintPageEventArgs)
        Dim line As String
        For Each row As DataGridViewRow In DataGridView1.Rows
            line = String.Empty
            For Each cell As DataGridViewCell In row.Cells
                line += cell.Value.ToString
            Next
            ' Render line to printer here
            Dim printFont As New System.Drawing.Font("Arial", 15, System.Drawing.FontStyle.Regular)
            Dim y As Integer
            y += 20
            ev.Graphics.DrawString(line, printFont, System.Drawing.Brushes.Black, 10, y)
        Next
    End Sub