I have an existing backoffice controller inheriting from UmbracoAuthorizedController that is used to preview a grid editor's output within the grid, by rendering a partial view with a model of type UmbracoViewPage<dynamic>

[IsBackOffice]
public class GridController : UmbracoAuthorizedController
{
    [HttpPost, ValidateInput(false)]
    public ActionResult Preview(string viewPath)
    {
        Request.InputStream.Seek(0, SeekOrigin.Begin);
        var json = new StreamReader(Request.InputStream).ReadToEnd();
        dynamic model = JsonConvert.DeserializeObject(json);

        return PartialView(viewPath, model);
    }
}

This has been working quite well, until the update to 7.4.0. The error I'm getting is:

[NullReferenceException: Object reference not set to an instance of an object.]     
Umbraco.Web.Mvc.UmbracoViewPage`1.SetViewData(ViewDataDictionary viewData)

Any ideas as to how I can fix this? Is there a way to change the internal state of the UmbracoContext so that my code no longer fails?