Проблема Xamarin Layout, как сделать кнопку буксировки внизу экрана

Я изучаю макет Xamarin и сталкиваюсь с приведенным ниже кодом:

<?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:MyApp.Validators"
             x:Class = "MyApp.MainPage">
    <StackLayout HorizontalOptions = "Fill"  >
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height = "*" ></RowDefinition>
                <RowDefinition Height = "*"></RowDefinition>
                <RowDefinition Height = "*"></RowDefinition>
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition  ></ColumnDefinition>
            </Grid.ColumnDefinitions>
            <Grid Grid.Row = "0" Grid.Column = "0">
                <Grid.RowDefinitions>
                    <RowDefinition Height = "*" ></RowDefinition>
                    <RowDefinition Height = "*"></RowDefinition>
                    <RowDefinition Height = "*"></RowDefinition>
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition></ColumnDefinition>
                </Grid.ColumnDefinitions>
                <Entry x:Name = "Username" Grid.Row = "0" Placeholder = "Username" PlaceholderColor = "Blue">
                    <Entry.Behaviors>
                        <local:UsernameValidatorBehavior></local:UsernameValidatorBehavior>
                    </Entry.Behaviors>
                </Entry>
                <Entry x:Name = "Password" Grid.Row = "1" Keyboard = "Numeric" Placeholder = "Password" PlaceholderColor = "Blue">
                    <Entry.Behaviors>
                        <local:PasswordValidatorBehavior></local:PasswordValidatorBehavior>
                    </Entry.Behaviors>
                </Entry>
                <Entry x:Name = "Email" Grid.Row = "2" Placeholder = "Email" PlaceholderColor = "Blue">
                    <Entry.Behaviors>
                        <local:EmailValidatorBehavior></local:EmailValidatorBehavior>
                    </Entry.Behaviors>
                </Entry>
            </Grid>
            <Grid VerticalOptions = "End" Grid.Row = "1" Grid.Column = "0">
                <Grid.RowDefinitions>
                    <RowDefinition Height = "Auto"></RowDefinition>
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width = "5*"></ColumnDefinition>
                    <ColumnDefinition Width = "5*"></ColumnDefinition>
                </Grid.ColumnDefinitions>
                <Button x:Name = "Submit" Grid.Row = "0" Grid.Column = "0" Text = "Submit" Clicked = "OnButtonClicked"></Button>
                <Button x:Name = "Cancel" Grid.Row = "0" Grid.Column = "1" Text = "Cancel" Clicked = "Cancel_Clicked"></Button>
            </Grid>
        </Grid>
    </StackLayout>
</ContentPage>

Мой вопрос в том, что я пытался использовать

<Grid VerticalOptions = "End" Grid.Row = "1" Grid.Column = "0">

чтобы две кнопки располагались внизу экрана, но вместо этого они отображались в середине экрана. Как решить эту проблему?

ОБНОВЛЕНИЕ1

Я изменил сетку, и теперь xaml выглядит следующим образом:

<?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:MyApp.Validators"
             x:Class = "MyApp.MainPage">
<StackLayout HorizontalOptions = "Fill"  >
<Grid VerticalOptions = "End">
<Grid.RowDefinitions>
<RowDefinition Height = "*" ></RowDefinition>
<RowDefinition Height = "*"></RowDefinition>
<RowDefinition Height = "*"></RowDefinition>
<RowDefinition Height = "*"></RowDefinition>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width = "*"></ColumnDefinition>
<ColumnDefinition Width = "*"></ColumnDefinition>
</Grid.ColumnDefinitions>
<Entry x:Name = "Username" Grid.Row = "0" Grid.ColumnSpan = "2" Placeholder = "Username" PlaceholderColor = "Blue">
<Entry.Behaviors>
<local:UsernameValidatorBehavior></local:UsernameValidatorBehavior>
</Entry.Behaviors>
</Entry>
<Entry x:Name = "Password" Grid.Row = "1" Grid.ColumnSpan = "2" Keyboard = "Numeric" Placeholder = "Password" PlaceholderColor = "Blue">
<Entry.Behaviors>
<local:PasswordValidatorBehavior></local:PasswordValidatorBehavior>
</Entry.Behaviors>
</Entry>
<Entry x:Name = "Email" Grid.Row = "2" Grid.ColumnSpan = "2" Placeholder = "Email" PlaceholderColor = "Blue">
<Entry.Behaviors>
<local:EmailValidatorBehavior></local:EmailValidatorBehavior>
</Entry.Behaviors>
</Entry>
<Button x:Name = "Submit" Grid.Row = "3" Grid.Column = "0" VerticalOptions = "End" Text = "Submit" Clicked = "OnButtonClicked"></Button>
<Button x:Name = "Cancel" Grid.Row = "3" Grid.Column = "1" 
VerticalOptions = "End" Text = "Cancel" Clicked = "Cancel_Clicked"></Button>
</Grid>   
</StackLayout>
</ContentPage>

Прямо сейчас есть только одна сетка с 4 строками и 2 столбцами. я добавил

VerticalOptions = "End" inside two buttons.

Но кнопок по-прежнему нет внизу экрана и более того, становится еще хуже.