How to use registerbackbuttonaction

i have 3 pages and i use: this.nav.setroot(page) to navigate…i’m try to use: back action and setroot(otherpage), any ??

this.platform.registerBackButtonAction(() => {
if(this.nav.canGoBack()){
this.presentToast(“Can go back…”);
}else{
this.presentToast(“Can’t back…”);
}
});

it’s always show: “Can’t Back”…
that plunker show the app: