app not running when changing the combobox value

Hello guys, i asked this question again but didn't seem to find an answer can you please healp me again.

im trying to run the application each time with a different comobox value (each time wit ha different worksheet).

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 problem.

Code:

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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 work1 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
        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 Button2.Click

        work1 = xlApp.Workbooks.Open(TextBox1.Text)
        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)

          work1.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

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That's not the way it works. Stick with the thread you already have open.