Invalid Use of Null in Access ComboBox - ms-access

Question

I am trying to use a ComboBox to append a query through a form I built. The Combobox should be optional, but I can't seem to get around the Invalid use of null error. Here is the code I have currently
Dim MyDB As DAO.Database
Dim qdef As DAO.QueryDef
Dim i As Integer
Dim strSQL As String
Dim strWhere As String
Dim strIN As String
Dim Box1 As String
Dim strBox1 As String
Dim flgSelectAll As Boolean
Dim varItem As Variant
Set MyDB = CurrentDb()
'General SQL Code
strSQL = "SELECT * FROM Test1"
'Build the IN string by looping through the listbox
For i = 0 To List6.ListCount - 1
If List6.Selected(i) Then
If List6.Column(0, i) = "_All" Then
flgSelectAll = True
End If
strIN = strIN & "'" & List6.Column(0, i) & "',"
End If
Next i
'Create the WHERE string, and strip off the last comma of the IN string
strWhere = " WHERE [Test1.Brand_Name] in " & _
"(" & Left(strIN, Len(strIN) - 1) & ")"
'Create the AND string
Box1 = Me.Combo8.Value
If IsNull(Me.Combo8.Value) Then
strBox1 = Nz(Me.Combo8.Column(0), "")
Else: strBox1 = " AND [Test1.Population] = '" & Box1 & "'"
End If
If Not flgSelectAll Then
strSQL = strSQL & strWhere & strBox1
End If
MyDB.QueryDefs.Delete "cpwg"
Set qdef = MyDB.CreateQueryDef("cpwg", strSQL)
'Open the query, built using the IN clause to set the criteria
DoCmd.OpenQuery "cpwg", acViewNormal
I have also tried
If IsNull(Box1) Or Box1 = "Null" Then
strBox1 = Nz(Me.Combo8.Column(0), "")
Else: strBox1 = " AND [Test1.Population] = '" & Box1 & "'"
End If

Answer 1

Try:
if isnull(me.combo8) then
Also, I Dont know how your combobox is populated, But Null is different than no data. Perhaps try
if me.combo8.value = "" then

Answer 2

IF IsNull(Trim(me.combo8)) Then
'Do Stuff
End if