Fix documnet that are missing default TEmplate in umbraco. Startup Event Handler

FixPublishedContentTemplates.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Umbraco.Core;
using Umbraco.Core.Models;
using Umbraco.Core.Services;

namespace FixDocumentTypeFuckUP
{
    public class FixPublishedContentTemplates : ApplicationEventHandler
    {
        protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
            var cs = applicationContext.Services.ContentTypeService;
            var c = applicationContext.Services.ContentService;

            foreach (var content in c.GetRootContent())
            {
                UpdateTemplate(content,c,cs);
            }

        }

        private void UpdateTemplate(IContent content, IContentService contentService, IContentTypeService cs)
        {
            foreach (var child in contentService.GetChildren(content.Id))
            {
                UpdateTemplate(child, contentService, cs);

                var pStatus = child.Published;

                var templateId = child.Template?.Id;
                
                if (null != templateId) continue;

                var firstOrDefault = child.ContentType.AllowedTemplates.FirstOrDefault();
                if (firstOrDefault == null) continue;

                child.Template = firstOrDefault;
                contentService.Save(child);
                if(pStatus == true)
                {
                    contentService.Publish(child, 0);
                }
            }
        }
    }
}