In Ember 1.x, how do I use a modal in willTransition()

I'm trying to open a modal dialog when someone tries to navigate away from a page with a form that isn't complete. I have the modal template built, but I can't figure out how to implement it. Here's what I have:

actions: {
    willTransition: function( transition ){
        var model = this.currentModel;

        if( model.get( 'isDirty' ) ){
            this.render( 'modal-confirmation', {
                into: 'application',
                outlet: 'modal'
            } );
        }
    }
}

NOTE: The dirty checking works and I can generate a prompt, but this modal thing is not working