Render ViewComponent as string from controller

Hi, I want to render viewcomponent as string in the controller itself. Not in the cshtml.

Here is the code I'm using now but it gets me a null reference exception in the the InvokeAsyncCore inside DefaultViewComponentHelper which implement IViewComponentHelper :

    public sealed class ViewComponentRender : IViewComponentRender
    {
        private readonly IViewComponentHelper _viewComponentHelper;

        public ViewComponentRender(IViewComponentHelper viewComponentHelper)
        {
            _viewComponentHelper = viewComponentHelper;
        }

        #region Implementation of IViewComponentRender

        public async Task<string> RenderToStringAsync(Type componentType, object arguments)
        {
//Exception in the line below :
            IHtmlContent htmlContent = await _viewComponentHelper.InvokeAsync(componentType, arguments);

            using (StringWriter stringWriter = new StringWriter())
            {
                htmlContent.WriteTo(stringWriter, HtmlEncoder.Default);
                return stringWriter.ToString();
            }
        }

        public async Task<string> RenderToStringAsync(Type componentType)
        {
//Exception in the line below :
            IHtmlContent htmlContent = await _viewComponentHelper.InvokeAsync(componentType);

            using (StringWriter stringWriter = new StringWriter())
            {
                htmlContent.WriteTo(stringWriter, HtmlEncoder.Default);
                return stringWriter.ToString();
            }
        }

        #endregion
    }