我试图创建一个自定义的List表单元素以显示从WCF服务获取的数据。以下是控件TrimSize的ViewModel的代码。 _trimSizeManager.GetRules（）返回我从WCF获取的对象列表-

    public class TrimSizeViewModel : ListViewModel,IBindingSettings,ICustomViewModel
    {
        private ITrimSizeManager _trimSizeManager;
        public TrimSizeViewModel()
        {
            _trimSizeManager = ServiceLocator.ServiceProvider.GetService<ITrimSizeManager>();
        }
        public string BindingToken { get; set; }
        public bool PrefillBindingValue { get; set; }
        public bool DisableonDataPrefill { get; set; }
        public string HelpText { get; set; }
        public string FieldAPIName { get; set; }
        public string DataParserClass => "ChoiceList";

        protected override void InitItemProperties(Item item)
        {
            base.InitItemProperties(item);
            this.InitBindingSettingsProperties(item);
            this.InitCustomViewModelProperties(item);
            Items.AddRange(_trimSizeManager.GetRules());
        }

        protected override void UpdateItemFields(Item item)
        {
            base.UpdateItemFields(item);
            this.UpdateBindingSettingsFields(item);
            this.UpdateCustomViewModelFields(item);
        }
    }

当我尝试在Sitecore表单部分的页面上加载该控件时，出现此错误stacktrace-

[NullReferenceException: Object reference not set to an instance of an object.]
   ProjectABC.Forms.Fields.TrimSize.TrimSizeViewModel.InitItemProperties(Item item) in C:\codebase\ProjectABC\ProjectABC\Fields\TrimSize\TrimSizeViewModel.cs:29
   Sitecore.ExperienceForms.Mvc.Pipelines.GetModel.CreateModel.Process(GetModelEventArgs args) +301
   (Object,Object ) +13
   Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args) +483
   Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName,PipelineArgs args,String pipelineDomain,Boolean failIfNotExists) +235
   Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName,String pipelineDomain) +21
Sitecore.Mvc.Pipelines.Pipelineservice.RunPipeline(String pipelineName,TArgs args) +194
   Sitecore.Mvc.Pipelines.Pipelineservice.RunPipeline(String pipelineName,TArgs args,Func`2 resultGetter) +160
   Sitecore.ExperienceForms.Client.Controllers.FormFieldController.RenderField(Guid fieldTypeId,Guid templateId) +625
   lambda_method(Closure,ControllerBase,Object[] ) +248
   system.web.Mvc.ReflectedactionDescriptor.Execute(ControllerContext controllerContext,IDictionary`2 parameters) +228
   system.web.Mvc.ControlleractionInvoker.Invokeactionmethod(ControllerContext controllerContext,actionDescriptor actionDescriptor,IDictionary`2 parameters) +34
   system.web.Mvc.Async.AsyncControlleractionInvoker.<BeginInvokeSynchronousactionmethod>b__39(IAsyncResult asyncResult,actioninvocation innerInvokeState) +38
   system.web.Mvc.Async.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult) +76
   system.web.Mvc.Async.AsyncControlleractionInvoker.EndInvokeactionmethod(IAsyncResult asyncResult) +41
   system.web.Mvc.Async.AsyncinvocationWithFilters.<InvokeactionmethodFilterAsynchronouslyRecursive>b__3d() +71
   system.web.Mvc.Async.<>c__DisplayClass46.<InvokeactionmethodFilterAsynchronouslyRecursive>b__3f() +387
   system.web.Mvc.Async.<>c__DisplayClass46.<InvokeactionmethodFilterAsynchronouslyRecursive>b__3f() +387
   system.web.Mvc.Async.<>c__DisplayClass46.<InvokeactionmethodFilterAsynchronouslyRecursive>b__3f() +387
   system.web.Mvc.Async.AsyncControlleractionInvoker.EndInvokeactionmethodWithFilters(IAsyncResult asyncResult) +42
   system.web.Mvc.Async.<>c__DisplayClass2b.<BeginInvokeaction>b__1c() +38
   system.web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeaction>b__1e(IAsyncResult asyncResult) +188
   system.web.Mvc.Async.AsyncControlleractionInvoker.EndInvokeaction(IAsyncResult asyncResult) +38
   system.web.Mvc.Controller.<BeginExecuteCore>b__1d(IAsyncResult asyncResult,ExecuteCoreState innerState) +29
   system.web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +73
   system.web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +52
   system.web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +39
   system.web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +38
   system.web.Mvc.MvcHandler.<BeginProcessRequest>b__5(IAsyncResult asyncResult,ProcessRequestState innerState) +43
   system.web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +73
   system.web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +38
   system.web.CallHandlerExecutionStep.system.web.HttpApplication.IExecutionStep.Execute() +648
   system.web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +213
   system.web.HttpApplication.ExecuteStep(IExecutionStep step,Boolean& completedSynchronously) +131

我已经在Sitecore内容编辑器中检查了TrimSize模板，模板ID指南，表单元素，表单元素guid和master，并且在Sitecore上似乎都正确。我找不到导致此错误的原因。 如果我用FieldViewModel替换ListViewModel，并注释诸如Addrange之类的列表模型方法并更改“视图”页面，则代码有效，并且我没有此异常。