<form #f="ngForm" (ngSubmit)="submit(f)">
 <div class="form-group">
  <label for="courseCategory"> Category </label>
  <select required ngModel name="courseCategory" #courseCategory="ngModel" id="courseCategory" class="form-control">
    <option value=""></option>
    <option *ngFor="let category of categories" [value]="category.id"> // line 16
      {{category.name}}
    </option>
  </select>
  <div class="alert alert-danger" *ngIf="courseCategory.touched && courseCategory.errors.required">
    Course category is required
  </div>
 </div>
</form>

在浏览器中,当我选择一个类别并按TAB(离开下拉列表)时,我在控制台上收到此错误：

CourseComponent.html:16 ERROR TypeError: Cannot read property
‘required’ of null
at Object.eval [as updateDirectives] (CourseComponent.html:20)

你能帮我找出导致这个错误的原因吗？

Boot Code 3.3.7已安装在VS代码中.