Work with the application each time with a different comoboxvalue

[highfly884]

hello guys , i need your help with my application.

im trying to run the application each time with a different comobox value. but the problem that the code works for the first time i click on the button 2 and when i try to change the combobox value run it with a different comobox value it shows this erreur
"Object reference not set to an instance of an object" on the ws1.

can you please show me how to run the app multiple times without this probleme .

Imports Microsoft.Office.Interop.Excel
Imports Microsoft.Office.Interop
Imports System.Windows.Forms.OpenFileDialog

Dim xlWorkSheet As Microsoft.Office.Interop.Excel.Worksheet
Dim xlApp As New Microsoft.Office.Interop.Excel.Application
Dim xlworkbook As Excel.Workbook
Dim ws1 As Excel.Worksheet
Dim cell As Excel.Range

Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles TextBox1.TextChanged

End Sub

    Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged

    End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        OpenFileDialog1.ShowDialog()
        TextBox1.Text = OpenFileDialog1.FileName
        xlworkbook = xlApp.Workbooks.Open(TextBox1.Text)

        For Each xlWorkSheet In xlApp.Sheets
            ComboBox1.Items.Add(xlWorkSheet.Name)
        Next
    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button3.Click

        ws1 = xlApp.Workbooks.Open(TextBox1.Text).Worksheets(ComboBox1.SelectedItem) ' here where the error shows
        cell=ws1.Cells(3, 5)
        cell.Interior.Color = System.Drawing.ColorTranslator.ToOle(System.Drawing.Color.Red)
            xlworkbook.Close()
            xlApp.Quit()
            System.Runtime.InteropServices.Marshal.ReleaseComObject(xlApp) : xlApp = Nothing
            System.Runtime.InteropServices.Marshal.ReleaseComObject(work1) : work1 = Nothing
            System.Runtime.InteropServices.Marshal.ReleaseComObject(ws1) : ws1 = Nothing
end sub

thank you guys for helping in advanced