Your code seems correct, it works fine for me..

Try to rebuild the external index in backoffice, and the if the error still occurs..

My simple example looks like this

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Umbraco.Cms.Core;
using Umbraco.Cms.Core.Models.PublishedContent;
using Umbraco.Cms.Web.Common.Controllers;

namespace UmbracoTestSite
{
    public class SearchController : UmbracoApiController
    {
        private readonly IPublishedContentQuery _publishedContentQuery;

        public SearchController(IPublishedContentQuery publishedContentQuery)
        {
            _publishedContentQuery = publishedContentQuery;
        }

        [HttpGet("search")]
        [ProducesResponseType(StatusCodes.Status200OK)]
        [ProducesResponseType(StatusCodes.Status404NotFound)]
        [ProducesResponseType(StatusCodes.Status500InternalServerError)]
        public ActionResult<IEnumerable<DocumentInfo>> GetBySearchTerm([FromQuery] string term)
        {
            if (string.IsNullOrWhiteSpace(term))
            {
                return NotFound("Missing parameter 'term'");
            }

            List<DocumentInfo> documentDetails;
            try
            {
                var publishedSearchResults = _publishedContentQuery.Search(term).ToList();

                documentDetails = publishedSearchResults.Select(x => PopulateDocumentInfo(x.Content))
                    .ToList();
            }
            catch (Exception ex)
            {
                //  _logger.LogError(ex, $"Get notifications for user {id} failed for type {notificationType}");
                return StatusCode(StatusCodes.Status500InternalServerError, "Get By Search Term failed");
            }

            return documentDetails;
        }

        private DocumentInfo PopulateDocumentInfo(IPublishedContent argContent)
        {
            return new DocumentInfo(argContent);
        }
    }

    public class DocumentInfo
    {
        public DocumentInfo(IPublishedContent content)
        {
            Name = content.Name;
        }

        public string Name { get; init; }
    }
}

Not 100% sure what fixed this issue but it seems that upgrading to the release version stopped this error.

is working on a reply...