Markup Extension: System.NullReferenceException

Hi guys, anyone can help me?

I'm getting this error and when I look my StackTrace, I've got this error: Xamarin.Forms.Xaml.MarkupExtensionParser.SetPropertyValue

Here is the code I'm using, just for test:


using System;

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace Essilor.Support
{
    public abstract class RelativeToView : IMarkupExtension
    {
        protected RelativeToView()
        {
            Factor = 1;
        }

        public string PropertyName { get; set; }

        public string ElementName { get; set; }

        public double Factor { get; set; }

        public double Constant { get; set; }

        public object ProvideValue( IServiceProvider serviceProvider )
        {
            var element = new ReferenceExtension { Name = ElementName }.ProvideValue( serviceProvider ) as View;
            if ( element != null )
            {
                var result = Constraint.RelativeToView( element, ( layout, view ) => DeterminePosition( view ) + Constant );
                return result;
            }
            return null;
        }

        protected virtual double DeterminePosition( VisualElement view )
        {
            var result = DetermineStart( view ) + DetermineExtent( view ) * Factor;
            return result;
        }

        protected abstract double DetermineExtent( VisualElement view );

        protected abstract double DetermineStart( VisualElement view );
    }
}


using System;

using Xamarin.Forms;

namespace Essilor.Support
{
    public class BelowOf: RelativeToView
    {
        protected override double DetermineExtent( VisualElement view )
        {
            return view.Height;
        }

        protected override double DetermineStart( VisualElement view )
        {
            return view.Y;
        }
    }
}

And here is the XAML which is causing the error:

<?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:support="clr-namespace:Essilor.Support;assembly=Essilor"
             x:Class="Essilor.Views.LoginView"
             BackgroundColor="White">
    <RelativeLayout Grid.Row="1"> 
        <Image  x:Name="logo"
            Aspect="Fill"
            RelativeLayout.XConstraint=
                 "{ConstraintExpression Type=Constant, 
                                        Constant=0}"
             RelativeLayout.YConstraint=
                 "{ConstraintExpression Type=Constant, 
                                        Constant=0}"
             RelativeLayout.WidthConstraint=
                 "{ConstraintExpression Type=RelativeToParent,
                                        Property=Width,
                                        Factor=1}"
             RelativeLayout.HeightConstraint=
                 "{ConstraintExpression Type=RelativeToParent,
                                        Property=Height,
                                        Factor=0.5}"
            Source="logo.png" />
        <Label
             x:Name="labelAccessKey"
             Text="Chave de Acesso"
             TextColor="Black"
             XAlign="End"
             RelativeLayout.XConstraint=
                 "{ConstraintExpression Type=Constant,
                                        Constant=0}"
             RelativeLayout.YConstraint=
                 "{ConstraintExpression Type=RelativeToView,
                                        ElementName=logo,
                                        Property=Height,
                                        Constant=40}"
             RelativeLayout.WidthConstraint=
                 "{ConstraintExpression Type=RelativeToParent,
                                        Property=Width,
                                        Factor=0.40}"
        />
        <Entry
             x:Name="accessKey"
             RelativeLayout.XConstraint=
                 "{ConstraintExpression Type=RelativeToView,
                                        ElementName=labelAccessKey,
                                        Property=Width,
                                        Constant=10}"
             RelativeLayout.YConstraint=
                 "{ConstraintExpression Type=RelativeToView,
                                        ElementName=labelAccessKey,
                                        Property=Y,
                                        Constant=-15}"
            RelativeLayout.WidthConstraint=
                 "{ConstraintExpression Type=RelativeToParent,
                                        Property=Width,
                                        Factor=0.5}"
        />

        <Label
             x:Name="labelPassword"
             Text="Senha"
             TextColor="Black"
             XAlign="End"
             RelativeLayout.XConstraint=
                 "{ConstraintExpression Type=Constant,
                                        Constant=0}"
             RelativeLayout.YConstraint=
                 "{ConstraintExpression Type=RelativeToView,
                                        ElementName=logo,
                                        Property=Height,
                                        Constant=80}"
             RelativeLayout.WidthConstraint=
                 "{ConstraintExpression Type=RelativeToParent,
                                        Property=Width,
                                        Factor=0.40}"
        />
        <Entry
             x:Name="password"
             RelativeLayout.XConstraint=
                 "{ConstraintExpression Type=RelativeToView,
                                        ElementName=labelPassword,
                                        Property=Width,
                                        Constant=10}"
             RelativeLayout.YConstraint=
                 "{ConstraintExpression Type=RelativeToView,
                                        ElementName=labelPassword,
                                        Property=Y,
                                        Constant=-15}"
             RelativeLayout.WidthConstraint=
                 "{ConstraintExpression Type=RelativeToParent,
                                        Property=Width,
                                        Factor=0.5}"
        />
        <Label
             x:Name="forgotPassword"
             Text="Esqueceu!"
             TextColor="Blue"
             XAlign="End"
             RelativeLayout.XConstraint=
                 "{ConstraintExpression Type=RelativeToView,
                                        ElementName=password,
                                        Property=X,
                                        Constant=0}"
             RelativeLayout.YConstraint=
                 "{support:BelowOf Type=RelativeToView,
                                        ElementName=password}"
        />

        <Button 
            Text = "Cadastrar"
            RelativeLayout.XConstraint=
                 "{ConstraintExpression Type=RelativeToParent,
                                        Property=Width,
                                        Factor=0.5}"
             RelativeLayout.YConstraint=
                 "{ConstraintExpression Type=RelativeToParent,
                                        Property=Height,
                                        Constant=-50}"/>

    </RelativeLayout>
</ContentPage>