macOS How to disable the Quit func under the mutilDocument environment ?

I haove a mutilDocument App , I found that when I open a window with the method : [NSApp beginSheet: [MultiWin window] modalForWindow: mainWindows modalDelegate: nil didEndSelector: nil contextInfo: nil]; The Quit func in mainmeny already highlight , this can confuse the user that the Quit app func can work under this vironment.I want to disable the Quit func under that , how to do? Thank you very much!