Thanks for reviewing my question!

I am trying to output a list of data from my database. and I want make the output changing when click different thing in the combobox.

I have the following code, it is almost the same for both code of them, however I can only run the second one which contains program(TEXT), but not the first one which contains year(INTEGER). I am wondering maybe because
yearcombobox
contains number instead of text which make them different. 



Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles yearComboBox.SelectedIndexChanged
        RankListBox.Items.Clear()
        yearcommand.CommandText = "select rank, universityname from program as p, ranking as r, university as u,institution as i where r.insid = i.insid and p.pid = r.pid and u.uid = r.uid and programName = '" & ProgramNameComboBox.Text & "' and i.institutionName = '" & institutionNameComboBox.Text & "' and r.year = " & yearComboBox.Text & " order by rank"


        Dim yearcheck As Object = yearcommand.ExecuteScalar()
        If yearcheck Is Nothing Then
            RankListBox.Items.Add("There is no ranking in current criterias!!!")
        Else
            Dim yeardatareader As OleDbDataReader = yearcommand.ExecuteReader()
            Do While yeardatareader.Read()
                RankListBox.Items.Add(yeardatareader.GetValue(0) & "-" & yeardatareader.GetValue(1))
            Loop
            yeardatareader.Close()
        End If


    End Sub


    Private Sub ProgramNameComboBox_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ProgramNameComboBox.SelectedIndexChanged
        'clear
        RankListBox.Items.Clear()
        'populate rank and university into listbox
        procommand.CommandText = "select rank, universityname,year from program as p, ranking as r, university as u, institution as i where i.insid = r.insid and p.pid = r.pid and u.uid = r.uid and programName = '" & ProgramNameComboBox.Text & "' and i.institutionName = '" & institutionNameComboBox.Text & "' and r.year = " & CyearComboBox.Text) & " order by rank"


        Dim procheck As Object = procommand.ExecuteScalar()
        If procheck Is Nothing Then
            RankListBox.Items.Add("There is no ranking in current criterias!!!")
        Else
            Dim prodatareader As OleDbDataReader = procommand.ExecuteReader()
            Do While prodatareader.Read()
                RankListBox.Items.Add(prodatareader.GetValue(0) & "   -   " & prodatareader.GetValue(1))
            Loop
            prodatareader.Close()
        End If


    End Sub
When I run the code and try to change the program combobox it works fine, however when i changed the year combobox, it will say error, and here is the screeshoot. screenshot.jpgIt said " System.NullReferenceException' and provide additional information: Object reference not set to an instance of an object.

What can I do, I really dont know whats wrong, I think I did the null exception already.

Thanks for your help!