Merge branch 'master' into simplify_authz

# Conflicts:
#	Jellyfin.Api/Auth/SyncPlayAccessPolicy/SyncPlayAccessHandler.cs

Jellyfin.Api/Controllers/ItemsController.cs

@@ -5,6 +5,7 @@ using Jellyfin.Api.Extensions;
 using Jellyfin.Api.Helpers;
 using Jellyfin.Api.ModelBinders;
 using Jellyfin.Data.Enums;
+using MediaBrowser.Common.Extensions;
 using MediaBrowser.Controller.Dto;
 using MediaBrowser.Controller.Entities;
 using MediaBrowser.Controller.Library;
@@ -240,7 +241,7 @@ public class ItemsController : BaseJellyfinApiController
         var isApiKey = User.GetIsApiKey();
         // if api key is used (auth.IsApiKey == true), then `user` will be null throughout this method
         var user = !isApiKey && userId.HasValue && !userId.Value.Equals(default)
-            ? _userManager.GetUserById(userId.Value)
+            ? _userManager.GetUserById(userId.Value) ?? throw new ResourceNotFoundException()
             : null;
 
         // beyond this point, we're either using an api key or we have a valid user
@@ -814,6 +815,11 @@ public class ItemsController : BaseJellyfinApiController
         [FromQuery] bool excludeActiveSessions = false)
     {
         var user = _userManager.GetUserById(userId);
+        if (user is null)
+        {
+            return NotFound();
+        }
+
         var parentIdGuid = parentId ?? Guid.Empty;
         var dtoOptions = new DtoOptions { Fields = fields }
             .AddClientFields(User)