Ссылка на объект не указывает на экземпляр объекта при выходе из программы - Visual Basic .NET

 ComboBox1.Items.Add(DataGridView2.Item(2, i).Value.ToString)

Private Sub StranaComboBox_SelectedIndexChanged_1(sender As Object, e As EventArgs) Handles StranaComboBox.SelectedIndexChanged
        Dim C As Integer
        Dim i As Integer
        Dim n As Integer
        Dim m As Integer
        Dim t As String
        n = TuragenstvoDataSet.Strany.Count
        t = StranaComboBox.SelectedValue
        ComboBox1.Items.Clear()
 
        For i = 0 To n
            If DataGridView1.Item(1, i).Value = t Then
 
                C = Convert.ToInt32(DataGridView1.Item(0, i).Value)
 
            End If
 
        Next
        m = TuragenstvoDataSet.Kurorty.Count
        For i = 0 To m
                If DataGridView2.Item(1, i).Value = C Then
 
                    ComboBox1.Items.Add(DataGridView2.Item(2, i).Value.ToString)
 
                End If
        Next
 
    End Sub
 
    Private Sub Button6_Click(sender As Object, e As EventArgs) Handles Button6.Click
        Form2.Show()
        Me.Hide()
 
    End Sub
 
    Private Sub Button7_Click(sender As Object, e As EventArgs) Handles Button7.Click
        ComboBox1.Items.Clear()
        Me.Close()
        Application.Exit()
        Exit Sub
    End Sub
End Class