Object reference not set to an instance of an object. when using for loop to store data in a grid view to a string

Question

Private Sub btnupload_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnupload.Click
    Dim Data As String = ""
    '  Dim i As Integer = 0
    '  For Each i As DataGridViewRow In DataGridView1.Rows
    For i As Integer = 0 To DataGridView1.RowCount - 1
        'For Each i As Integer =0 to DataGridView1.
        Data = DataGridView1.Rows(i).Cells(0).Value.ToString + "~" + DataGridView1.Rows(i).Cells(1).Value.ToString + "~" + DataGridView1.Rows(i).Cells(2).Value.ToString + "~" + DataGridView1.Rows(i).Cells(3).Value.ToString + "*"

    Next

End Sub