pushasync-is-not-supported-globally-on-android-please-use-a-navigationpage

I am trying to visualize the data of a listView in the MainPage (main) by passing it from another page but it gives me this error ... pushasync-is-not-supported-globally-on-Android-please-use-a-navigationpage .

In the App.cs I have this:

public partial class App : Application
{
    public App ()
    {
        InitializeComponent();

        MainPage = new NavigationPage(new MainPage());
    }

And in another class ...

public partial class AnadirConjuntoPage : ContentPage
{
    var m = new MainPage();
    await Navigation.PushAsync(m);
}

I tried to change this but it does not let me pass the variable m:

await Navigation.PushAsync(new ContentPage());

If I put this, I do not get an error but I do not get the toolbar

await Navigation.PushModalAsync(m);