I have created a custom block in which I have added a property that is supposed to be I think an image picker where you can pick the focal point, but when I edit that block in the CMS, I am only shown a button that says "Remove focal point" with a red dot and no image picker to add a focal point to. What could I be doing wrong?

The package that I am using:

https://nuget.optimizely.com/package?id=ImageResizer.Plugins.EPiFocalPoint

Screenshot of the focal point field not showing an image picker:

Code snippet of my custom block with the focal point property:

[ContentType(
    DisplayName = "Form container",
    Description = "This is a custom container",
    GUID = HopFormContainerBlockConstants.GUID,
    GroupName = Constants.FormElementGroup_Container,
    Order = 4000)]
[SiteImageUrl("~/Static/gfx/blocks/Form_Container_Block.png")]
public class HopFormContainerBlock : FormContainerBlock
{
    [Display(GroupName = SystemTabNames.Content, Order = 10)]
    [UIHint(UIHint.Image)]
    [CultureSpecific]
    [BackingType(typeof(PropertyFocalPoint))]
    public virtual FocalPoint FocalPoint { get; set; }
}