Validation 在登录测试中，量角器测试失败

我正在量角器上运行一个测试，不知道为什么它会失败——而一个类似的测试通过了。 我正在测试一个注册窗口。我希望在firstname或lastname丢失时看到验证警告

这是标记：

<section class="form_field"> <input data-ng-model="userInfo.userFirstName" data-ng-maxlength="30" data-ng-minlength="2" name="first" type="text" class="form-control ng-pristine ng-animate ng-valid-remove ng-invalid-add ng-invalid-required-add ng-valid-maxlength-add ng-valid-remove-active ng-invalid ng-invalid-add-active ng-invalid-required ng-invalid-required-add-active ng-valid-maxlength ng-valid-maxlength-add-active ng-valid-minlength" placeholder="First Name*" required="" style=""> <div class="field_error" ng-show="registerForm.first.$invalid"> <span data-ng-show="registerForm.first.$error.required" class="ng-binding">Required</span> <span data-ng-show="registerForm.first.$error.minlength" class="ng-hide">Cannot be shorter than 2 letters</span> <span data-ng-show="registerForm.first.$error.maxlength" class="ng-hide">Cannot be longer than 30 letters</span> </div> </section>
<section class="form_field"> <input data-ng-model="userInfo.userLastName" data-ng-maxlength="30" data-ng-minlength="2" name="last" type="text" class="form-control ng-valid-maxlength was_focused ng-animate ng-dirty ng-valid-minlength ng-invalid ng-invalid-required" placeholder="Last Name*" required="" style=""> <div class="field_error" ng-show="registerForm.last.$invalid"> <span data-ng-show="registerForm.last.$error.required" class="ng-binding">Required</span> <span data-ng-show="registerForm.last.$error.minlength" class="ng-hide">Cannot be shorter than 2 letters</span> <span data-ng-show="registerForm.last.$error.maxlength" class="ng-hide">Cannot be longer than 30 letters</span> </div> </section>

执行第二个测试时（仅输入lastname和expect firstname警告），失败：

browser.findElement(by.name('last')).sendKeys("last");
//pause for debug
//browser.pause();
  browser.driver.findElement(By.xpath("//button[text() = 'CONTINUE']")).click();
  browser.sleep(8000);
  expect($('[data-ng-show="registerForm.first.$error.required"]').isDisplayed()).toBeTruthy();

我得到这个错误：

 ElementNotVisibleError: element not visible

任何帮助都将不胜感激

谢谢

 ElementNotVisibleError: element not visible