Minor change to get it to compile.

Emby.Server.Implementations/ApplicationHost.cs

@@ -1319,7 +1319,7 @@ namespace Emby.Server.Implementations
         /// <inheritdoc/>
         public string GetLoopbackHttpApiUrl()
         {
-            if (NetworkManager.IsIP6Enabled)
+            if (NetManager.IsIP6Enabled)
             {
                 return GetLocalApiUrl("::1", Uri.UriSchemeHttp, HttpPort);
             }

Jellyfin.Api/Controllers/StartupController.cs

@@ -72,9 +72,9 @@ namespace Jellyfin.Api.Controllers
         [ProducesResponseType(StatusCodes.Status204NoContent)]
         public ActionResult UpdateInitialConfiguration([FromBody, Required] StartupConfigurationDto startupConfiguration)
         {
-            _config.Configuration.UICulture = startupConfiguration.UICulture;
-            _config.Configuration.MetadataCountryCode = startupConfiguration.MetadataCountryCode;
-            _config.Configuration.PreferredMetadataLanguage = startupConfiguration.PreferredMetadataLanguage;
+            _config.Configuration.UICulture = startupConfiguration.UICulture ?? string.Empty;
+            _config.Configuration.MetadataCountryCode = startupConfiguration.MetadataCountryCode ?? string.Empty;
+            _config.Configuration.PreferredMetadataLanguage = startupConfiguration.PreferredMetadataLanguage ?? string.Empty;
             _config.SaveConfiguration();
             return NoContent();
         }

MediaBrowser.Model/Configuration/ServerConfiguration.cs

@@ -1,4 +1,3 @@
-#nullable enable
 #pragma warning disable CS1591
 #pragma warning disable CA1819