例外：未捕获（承诺）：TypeError：无法读取 null 的属性“touched”

我正在尝试在我的 Angular2 应用程序中使用响应式表单，但我遇到了如下异常

例外：未捕获（承诺）：TypeError：无法读取 null 的属性“touched”类型错误：无法读取 null 的属性“touched”

我的示例代码如下

import { Component, OnInit } from '@angular/core';
import { NgForm } from '@angular/forms';
import {FormGroup, FormControl} from '@angular/forms';
import { Customer } from './customer';

@Component({
moduleId:module.id,
templateUrl: 'sign-up.reactiveForms.html'
})

export class SignupComponent  {

customer: Customer= new Customer();


customerForm : FormGroup; //it assosciate html element with this root model

ngOnInit() : void {


   this.customerForm=new FormGroup({

        firstName: new FormControl(),
         lastName: new FormControl(),
         email: new FormControl(),
         sendCatalog: new FormControl(true)

   });
}

save() {
    console.log(this.customerForm);
  }

}

sign-up.reactiveForms.html 文件如下

            <!-- First Name input -->

            <div class="form-group"
                [ngClass]="{'has-error': (customerForm.get('firstName').touched ||customerForm.get('firstName').dirty) && !customerForm.get('firstName').valid }">
                <label class="col-md-2 control-label" 
                       for="firstNameId">First Name</label>

                <div class="col-md-8">
                    <input class="form-control" 
                           id="firstNameId" 
                           type="text" 
                           placeholder="First Name (required)" 
                           required 
                           minlength="3"
                           formControlName="firstName"
                             />
                    <span class="help-block" *ngIf="(customerForm.get('firstName').touched || customerForm.get('firstName').dirty) && customerForm.get('firstName').errors">
                        <span *ngIf="customerForm.get('firstName').errors.required">
                            Please enter your first name.
                        </span>
                        <span *ngIf="customerForm.get('firstName').errors.minlength">
                            The first name must be longer than 3 characters.
                        </span>
                    </span>
                </div>
            </div>

我想弄清楚为什么 touch 属性为空。