我有一個 Angular 8 應(yīng)用程序。但是我收到這個錯誤：在一個組件上。它是一個組件中的一個組件。如果組件為空，則將顯示錯誤，如果組件不為空，則不會顯示錯誤:4200/dossier-dossier-module-ngfactory.js:186 ERROR TypeError: Cannot read property 'length' of null    at DossierCorrespondenceItemComponent.push../src/app/dossier/dossier-correspondence-item/dossier-correspondence-item.component.ts.DossierCorrespondenceItemComponent.ngOnInit (:4200/dossier-dossier-module-ngfactory.js:100)    at checkAndUpdateDirectiveInline (vendor.js:37850)    at checkAndUpdateNodeInline (vendor.js:46063)    at checkAndUpdateNode (vendor.js:46025)    at debugCheckAndUpdateNode (vendor.js:46659)    at debugCheckDirectivesFn (vendor.js:46619)    at Object.updateDirectives (:4200/dossier-dossier-module-ngfactory.js:189)    at Object.debugUpdateDirectives [as updateDirectives] (vendor.js:46611)    at checkAndUpdateView (vendor.js:46007)    at callViewAction (vendor.js:46248)在這個組件上：  <app-dossier-correspondence-item    [item]="single"    (goBack)="goBack($event)"    *ngIf="showingSingle">  </app-dossier-correspondence-item></app-vital10-page>這是 ts 代碼：import { Component, OnInit } from '@angular/core';import { HealthAPIService } from '../../shared/health-api/health-api.service';import { DossierEntry } from '../../interfaces/dossier/dossier-entry.interface';@Component({  selector: 'app-dossier-correspondence',  templateUrl: './dossier-correspondence.component.html',})export class DossierCorrespondenceComponent implements OnInit {  allCorrespondence: Array<DossierEntry>;  correspondenceEntries: Array<DossierEntry>;  attachmentEntries: Array<DossierEntry>;  message = '';  emptyMessage = 'Geen correspondentie.';  errorMessage = 'Er ging iets mis met de connectie. Probeer over enkele minuten nogmaals.';  correspondenceLoaded = false;  showingSingle = false;  single: DossierEntry;那么如何解決這個問題呢？謝謝你。所以會很好地解決這個問題