Difference between PushAsync and PushModalAsync ?

I'm getting Error and found some solution, I don't is this good solution or not.

Error : System.InvalidOperationException: PushAsync is not supported globally on iOS, please use a NavigationPage.

try{
Navigation.PushAsync (new LoginScreen ()); // Exception come and crash the application
}catch(Exception exception){
Navigation.PushModalAsync(new LoginScreen ()); // Works
}

But after did this implementation on next screen same Exception Come again... Is their any good solution or should I do on every page ?