Vba UserForm combobox items don't apear till i click on the userform

I have a command button, when i click it, it shows a customized userform which contains a combobox with items taken from a sheet cells. the user is suppose to press the command then the userform appears, he needs to select one item from the list, the item is a sheet name in different workbook, accordingly it will import the data from that sheet. however i am facing a problem: when the userform appears, i click on the combobox it shows no items, then if i click once on the userform then check the combobox again it shows the items correctly, if i click again on the userform, the combobox items will be doulbled! if i selected an item it works perfectly. So what i want to achieve is: i want the combobox to show the items directly without clicking on the userform. the code is this:

In the userform:

Private Sub UserForm_Click()



Dim mlf As Workbook

Dim adad As Long

Dim mada As String

Dim lastRow As Long

Set mlf = ActiveWorkbooklastRow = Sheet3.Cells(Rows.Count, 1).End(xlUp).Row

For adad = 1 To lastRow

mada = Sheet3.Cells(adad, 1)

With ComboBox1

        .AddItem mada

End With

Next

    End Sub


Private Sub cmdOkay_Click()

 'Verify that an item was selected

If Me.ComboBox1.BoundValue = vbNullString Then

    MsgBox "You did not choose an item!", vbOKOnly

    Exit Sub

Else

    MsgBox "You have selected " & Me.ComboBox1.BoundValue, vbOKOnly

    Sheet3.Cells(1, 2) = Me.ComboBox1.BoundValue


End If

   Unload Me 

End Sub

In the CommandButton

Dim testbook As Workbook

Set testbook = ThisWorkbook

Set database = Workbooks.Open(Filename:=myPath & myFile)

testbook.Worksheets("Sheet4").Range("A1:B5").Clear

With UserForm1
.Caption = "Settings of the test"
End With

Dim lo As Integer

For lo = 1 To database.Sheets.Count

     testbook.Sheets("Sheet4").Cells(lo, 1) = database.Worksheets(lo).Name

Next

UserForm1.Show