Custom controls in Xamarin Previewer not showing

HI

I'm trying to do something extremely simple that just isn't working and I can't find anything online to help.

I've got an iOS project in Xamarin Forms and I'm trying to create a StackLayout with custom controls in it. Each custom control is effectively a message, so (even though it's not), imagine something like WhatsApp where each message is a custom control. I've been trying to ages to try and see my custom control in the previewer (I've got a slight layout issue that I'm trying to resolve and I'd like to see my controls in the Previewer).

So to try and simplify it, I've created a bare bones BoxView custom control like this:

<?xml version="1.0" encoding="UTF-8"?>
<BoxView xmlns="http://xamarin.com/schemas/2014/forms"
     xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
     x:Class="Mk.Views.ClassNews.MyView" BackgroundColor="Gray">
</BoxView>

And the code behind looks like this:

using System;
using System.Collections.Generic;
using Xamarin.Forms;

namespace Mk.Views.ClassNews
{
    public partial class MyView : BoxView
    {
        public MyView()
        {
            InitializeComponent();
        }
    }
}

In the Previewer, my custom control looks as it should, just a grey screen. But when I include this custom control in another view's layout (like in a StackLayout for instance) it displays when I run the app, but it shows nothing in the Previewer.

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
     xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
     x:Class="Mk.Views.ClassNews.TestPage"
             xmlns:cs="clr-namespace:Mk.Views.TestPage">
    <ContentPage.Content>
        <cs:MyView/>
    </ContentPage.Content>
</ContentPage>

And the code behind:

using System;
using System.Collections.Generic;

using Xamarin.Forms;

namespace Mk.Views.ClassNews
{
    public partial class TestPage : ContentPage
    {
        public TestPage()
        {
            InitializeComponent();
        }
    }
}

Running the app, the screen goes gray but the Previewer just stays white. Any ideas as to why?

Thanks