PropertyDataDataSource.cs

using System.Collections.Generic;
using System.Linq;
using Umbraco.Community.Contentment.DataEditors;
using Umbraco.Core.PropertyEditors;
using Umbraco.Web;

namespace MyWebsite.DataSources
{
    public class PropertyDataDataSource : IDataListSource
    {
        private readonly IUmbracoContextAccessor _umbracoContextAccessor;

        public PropertyDataDataSource(IUmbracoContextAccessor umbracoContextAccessor)
        {
            _umbracoContextAccessor = umbracoContextAccessor;
        }

        public string Name => "Umbraco Property Data";

        public string Description => "Data source for umbraco property data coming from the website root node.";

        public string Icon => "icon-globe";

        public OverlaySize OverlaySize => OverlaySize.Small;

        public Dictionary<string, object> DefaultValues => new Dictionary<string, object>();

        public IEnumerable<ConfigurationField> Fields => new ConfigurationField[]
        {
            new ConfigurationField
            {
                Key = "propAlias",
                Name = "Property",
                Description = "Select the property to populate the data source with.",
                View = "~/umbraco/views/propertyeditors/entitypicker/entitypicker.html",
                Config = new Dictionary<string, object>
                {
                    { "multiple", "0" },
                    { "entityType", "PropertyType" },
                    { "publishBy", "alias" },
                }
            }
        };

        public IEnumerable<DataListItem> GetItems(Dictionary<string, object> config)
        {
            if (config.TryGetValue("propAlias", out var tmp) && tmp is string alias && string.IsNullOrWhiteSpace(alias) == false)
            {
                var content = _umbracoContextAccessor.UmbracoContext.Content.GetAtRoot().First();
                if (content != null)
                {
                    var propertyValue = content.Value<string[]>(alias);
                    if (propertyValue != null)
                    {
                        return propertyValue.Select(x => new DataListItem { Name = x, Value = x });
                    }
                }
            }

            return Enumerable.Empty<DataListItem>();
        }
    }
}

Agent Pitcher, we meet again! 🕵️‍♂️ // @LottePitcher @greystate

Looking at how I've done this elsewhere in Contentment, here's an example:
https://github.com/leekelleher/umbraco-contentment/blob/4.2.0/src/Umbraco.Community.Contentment/DataEditors/SocialLinks/SocialLinksConfigurationEditor.cs#L103

Would this not work for you? 🤔
View = _ioHelper.ResolveRelativeOrVirtualUrl("~/umbraco/views/propertyeditors/entitypicker/entitypicker.html"),

Previously, (e.g. v8) you could omit the ~/umbraco/ bit, as that was already in the context the "umbraco" directory, but in this .NET Core world, the paths are somewhat different. I'm not yet sure what the context of the "root" directory is, e.g. Content directory or the Web (wwwroot) directory. 🤷 That's why I use the ResolveRelativeOrVirtualUrl extension method to sort it out for me. 😬

Thanks @leekelleher as of course I just needed the umbraco/ in the view path. Good job @greystate isn't paying me for this, otherwise that might have been embarrassing 😆

I still get an error but I now see that's from the Config dictionary field property (as lifted from v8), so I can make progress with investigating that now. I think I was thinking this might be a problem caused by RCLs or something... But clearly not! 🙏