combox.Items.Assign(Printer.Printers);在默认打印机不可用的时候，提示错误，(50分)

运行程序创建窗体时，
begin
Application.Initialize;
Application.CreateForm(TDM, DM);
Application.CreateForm(TfrmMain, frmMain);
Application.CreateForm(TfrmPrnSet, frmPrnSet);//这个窗体的FormCreate事件里有combox.Items.Assign(Printer.Printers);
Application.CreateForm(TfrmPrnYL, frmPrnYL);
Application.Run;
end.

如何屏蔽？