Variable not defined error

Hi,
I copied code from another sheet into my own and I keep getting a variable not defined on "Listbox1" I'm not sure why. Can someone help me? Here is the code:

Option Explicit

Dim rg As Range
Private Sub Worksheet_Activate()
   Set rg = Sheets(1).Range("A6:A25")      '<< Update Here : Range with list names
   ' OR
   'Set rg = Sheets(1).Range("A6:A" & Sheets(1).Range("A" & Rows.Count).End(xlUp).Row)     'dynamic range
End Sub

Sub PopulateListBox()
   Dim ar, i As Integer
   
   Application.ScreenUpdating = False
   
   If rg Is Nothing Then Worksheet_Activate
   
   rg.EntireRow.Hidden = False      'By default all series are visible
   ar = rg.Value
   With ListBox1
      .List = ar
      For i = 0 To .ListCount - 1
          .Selected(i) = True
      Next i
   End With

   Application.DisplayAlerts = True
End Sub

Private Sub ListBox_Mouseup(ByVal Button As Integer, ByVal Shift As Integer, ByVal X As Single, ByVal Y As Single)
'When listbox is clicked, refresh series
   Dim i As Integer
   Application.ScreenUpdating = False
   With ListBox1
      For i = 0 To .ListCount - 1
         rg.Offset(i).EntireRow.Hidden = Not .Selected(i)
         Debug.Print i; .Selected(i)
      Next i
   End With
   Application.ScreenUpdating = True
End Sub