Error message if field is empty access

Hello,

I have a form that serves to add data to a table, using 4 text boxes and a final button.
The form works very well, but I would like it so that if a field is empty when validating the addition, the record is not created and an error message appears.

Here is my code:

Private Sub btn_ajouter_Click()

Set db = CurrentDb
Set rs = db.OpenRecordset("Stock_atelier")

rs.AddNew

rs!Référence = Me.txt_reference
rs!Fournisseur = Me.txt_fournisseur
rs!Quantité = Me.txt_quantité
rs!Type = Me.txt_outil

rs.Update

rs.Close
db.Close

MsgBox "Référence ajoutée avec succès "
txt_reference = ""
txt_fournisseur = ""
txt_quantité = ""
txt_outil = ""

Set rs = Nothing
Set db = Nothing

End Sub

I tried to add the following lines:

If IsNull(Me.txt_reference) Then
 MsgBox ("Veuillez renseigner toutes les cases")
ElseIf IsNull(Me.txt_fournisseur) Then
 MsgBox ("Veuillez renseigner toutes les cases")
ElseIf IsNull(Me.txt_quantité) Then
 MsgBox ("Veuillez renseigner toutes les cases")
ElseIf IsNull(Me.txt_outil) Then
 MsgBox ("Veuillez renseigner toutes les cases")
End If

But nothing works (maybe I didn't put them in the right place or who knows).

Could you help me??
Thanks in advance.