can't disable button on system form ("0")

Hi everybody,

i have a problem.i want to make an add-on which should work like that:

-   disabled the button system "close"  on the form system AR Invoice or

-   disabled  the "No" button  and "Cancel" button of  the dialog box when i try to close  the AR Invoice form.

this is a part of code  where :

SAPbouiCOM.Form frm = Application.SBO_Application.Forms.Item("0"); ------------- it is here that the message error occurs

            frm.Items.Item("2").Enabled = false;  // to disable "No" button

           frm.Items.Item("3").Enabled = false;   // to disable "Cancel " button

the message errors show :  Form - Invalid Form.

Can you help me please ?