Cannot use async in closing method

Question

I make a method called Instance that allow me to have a single instance of the Settings window, like this:

    public static async Task<Settings> Instance()
    {
        if (AppWindow == null)
        {
            AppWindow = new Settings();

            AppWindow.Closing += async (x, y) =>
            {
                bool close = await AppWindow.CheckSettings();
                y.cancel = (close) ? true : false;
                AppWindow = null;
            };
        }

        return AppWindow;
    }

the CheckSettings have this structure:

private async Task<bool> CheckSettings()
{   
     //just as example
    return true;
}

the method Instance() tell me that there is no await operator inside. Why happen this?

I need to ask also other questions:

1. Can this logic used inside a property instead of Instance method? How?
2. Is possible close the window without implement a Task<bool>

UPDATE

based on the helpful answer and comments on this great community I have edited the method as this (now is a property):

    public static Settings Instance
    {
        get
        {
            if (AppWindow == null)
            {
                AppWindow = new Settings();

                AppWindow.Closing += async (x, y) =>
                {
                    bool close = await AppWindow.CheckSettings();
                    y.Cancel = close;

                    //AppWindow.Close();
                    //AppWindow = null;
                };
            }

            return AppWindow;
        }
    }

the problem is that the Cancel does not await the CheckSettings()

Answer 1

Set the Cancel property to true before you call your async method:

public static Settings Instance
{
    get
    {
        if (AppWindow == null)
        {
            AppWindow = new Settings();
            //attach the event handler
            AppWindow.Closing += AppWindow_Closing;
        }
        return AppWindow;
    }
}

private static async void AppWindow_Closing(object sender, System.ComponentModel.CancelEventArgs e)
{
    e.Cancel = true;

    //call the async method
    bool close = await AppWindow.CheckSettings();
    if (close)
    {
        AppWindow win = (AppWindow)sender;
        //detach the event handler
        AppWindow.Closing -= AppWindow_Closing;
        //...and close the window immediately
        win.Close();
        AppWindow = null;
    }
}