Xamarin - PushAsync fails when using ContentPage.BindingContext - xamarin

Question

I have two pages, the main page, which displays informations, and a login page, which should be shown, if I'm not logged in. The main page get it's informations via the ContentPage.BindingContext attribute. I'm using following method to show the login screen: Application.Current.MainPage.Navigation.PushModalAsync(new LoginPage());
When I run the app with the ContentPage.BindingContext, the code fails with System.NullReferenceException: 'Object reference not set to an instance of an object.'. If I remove the BindingContext, start the app, and add the BindingContext with Xamarin Hot Reload, the login screen will be displayed.
Here is an excerpt from the code:
MainPage.xaml
<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:MyCalendar"
xmlns:iOsSpecific="clr-namespace:Xamarin.Forms.PlatformConfiguration.iOSSpecific;assembly=Xamarin.Forms.Core"
x:Class="MyCalendar.MainPage"
iOsSpecific:Page.UseSafeArea="True">
<ContentPage.BindingContext>
<local:MyViewModel/>
</ContentPage.BindingContext>
</ContentPage>
MyViewModel.cs
public class MyViewModel : INotifyPropertyChanged
{
private readonly MyData data;
public MyViewModel()
{
data = new MyData();
}
protected void RaisePropertyChanged(string name)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
}
}
MyData.cs
public class MyData
{
public MyData()
{
// Fetching data .....
case Error:
Application.Current.MainPage.Navigation.PushModalAsync(new LoginPage(), true);
break;
case OK:
// Execute code
break;
default:
Console.WriteLine("SOME ERROR");
break;
}
}
Is there any way to show the login page?

Answer 1

The problem was that the BindingContext, when set in the .xaml, still executed before the MainPage was (fully) initialized. Accordingly, an NPE always came.
I was able to fix it simply by adding a new line MainPage.BindingContext = new MyViewModel(); in the App.xaml.cs and removing the BindingContext from the .xaml. This ensured that the BindingContext is only executed when the MainPage is completely loaded.