If I click stylesheet i'm getting a this type of error.

× Server error: Contact administrator, see log for full details. Failed to retrieve data for child nodes undefined × {"errorMsg":"Failed to retrieve data for child nodes undefined","data":{"Message":"An error has occurred.","ExceptionMessage":"Object reference not set to an instance of an object.","ExceptionType":"System.NullReferenceException","StackTrace":" at umbraco.loadStylesheets.Render(XmlTree& tree)\r\n at Umbraco.Web.Trees.ApplicationTreeExtensions.TryGetXmlTree(ApplicationTree appTree, String id, FormDataCollection formCollection)\r\n at Umbraco.Web.Trees.ApplicationTreeExtensions.TryLoadFromLegacyTree(ApplicationTree appTree, String id, FormDataCollection formCollection, UrlHelper urlHelper, String currentSection)\r\n at Umbraco.Web.Trees.LegacyTreeController.GetTreeNodes(String id, FormDataCollection queryStrings)\r\n at Umbraco.Web.Trees.TreeControllerBase.GetNodes(String id, FormDataCollection queryStrings)\r\n at lambda_method(Closure , Object , Object[] )\r\n at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>cDisplayClass10.9(Object instance, Object[] methodParameters)\r\n at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, CancellationToken cancellationToken)\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Web.Http.Controllers.ApiControllerActionInvoker.