Why does it not work without question box ?

Hello all, 

I'm really happy to finally find this forum !

I've picked some code here which helped me a lot today ... but still 

The following code works : 

MessageBox.Show("Nous faisons paraître ici une fenêtre, car sans elle un message d'erreur est retourné.", "OK");
Form f = Form.GetFormFromUID(customize.B1Connector.GetB1Connector().Application.Forms.ActiveForm.UniqueID);
string NbPalettes = TextEdit.GetFromUID(f, "U_Palettes").Value;

But this one doesn't :

Form f = Form.GetFormFromUID(customize.B1Connector.GetB1Connector().Application.Forms.ActiveForm.UniqueID);
string NbPalettes = TextEdit.GetFromUID(f, "U_Palettes").Value;

The only difference is the MessageBox.Show  ..

Isn't odd ? 

The error message given by the second code is : 

Item - invalid item [66000-3]. Form Unique Id : 'F_285', Item Unique Id : 'U_Palettes'

I want to make the second one ( without MessageBox.Show command ) work.

What have I to do ? 

Thanks.