PreviousPageModel is null

Question

I have a Xamarin.Forms application with freshmvvm framework. According to the documentation, I can use PreviousPageModel property of FreshBasePageModel base class to access data of the PageModel I navigated from. I navigate like this:

    public FirstPageModel()
    {
        _validator = new CalculatorValidator();
        CalculateCommand = new Command(execute: () => 
        {
            ValidationResult = _validator.Validate(this);
            RaisePropertyChanged(nameof(ValidationResult));

            if (ValidationResult.IsValid)
            {
                CoreMethods.PushPageModel<SecondPageModel>();
            }
        });
    }

The navigation happens, but in the SecondPageModel constructor the PreviousPageModel is null:

    public SecondPageModel()
    {
        _previousModel = (FirstPageModel)PreviousPageModel;
    }

What am I doing wrong?

Thank you.

EDIT:

I also tried:

    public FirstPageModel()
    {
        _validator = new CalculatorValidator();
        CalculateCommand = new Command(Calculate);
    }

    private void Calculate()
    {
        ValidationResult = _validator.Validate(this);
        RaisePropertyChanged(nameof(ValidationResult));

        if(ValidationResult.IsValid)
        {
            CoreMethods.PushPageModel<SecondPageModel>(this);
        }
    }

Answer 1

I got the answer here:

https://forums.xamarin.com/discussion/comment/365262/#Comment_365262

The PreviousPageModel is null because it hasn't been set in the constructor. Place your code in the ViewIsAppearing lifecycle event, then you will get the correct previous model:

protected override void ViewIsAppearing(object sender, EventArgs e)
{
    base.ViewIsAppearing(sender, e);

    _previousModel = (FirstPageModel)PreviousPageModel;
}