Greetings to all

! I am new to openDCL, and I may need some help in the development.
I have a modal dialog with a combobox inside. My task is to add a list of text into the combobox. However, the combobox seems unchanged, and a runtime error occurs after closing the dialog: "Error: NIL value not allowed; Function: dcl-Control-SetList". The reason could be the inproper settings to the modal dialog, since when I executed the same code in a modeless dialog, it worked. But I don't know what is going wrong in my modal dialog. I appreciate it if anyone can give some help.
Here is my code:
(dcl-Form-Show New/SettingDlg)
(setq testlst '("1" "2" "3"))
(dcl-Control-SetList New/SettingDlg/Layer1Cmbbox testlst)