[[NSApplication sharedApplication] runModalForWindow:mainWindow];

- (void)windowWillClose:(NSNotification *)notification {
   [[NSApplication sharedApplication] stopModal];
}

- (void)showSheet:(id)sender
{
    [NSApp beginSheet:yourModalWindow
        modalForWindow:windowThatSheetIsAttachedTo
        modalDelegate:self
        didEndSelector:@selector(sheetDidEnd:returnCode:contextInfo:)
           contextInfo:nil];
}

- (void)sheetDidEnd:(NSWindow *)sheet returnCode:(NSInteger)returnCode contextInfo:(void *)contextInfo
{
    [sheet orderOut:self];
    [NSApp endSheet:sheet];
}

[[NSApplication sharedApplication] stopModal];

window.beginSheet(self.uiSettingsPanel, completionHandler: {response in
        NSLog("Finished sheet, response: (response)")
})

@IBAction func buttonOK(_ sender: NSButton) {
    self.sheetParent!.endSheet(self, returnCode: .OK)
}