ember-tether remove with isShowing error

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  isShowing:false,
  actions:{
   clicked(){
    this.toggleProperty('isShowing');
   }
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

templates.application.hbs

{{my-component}}

templates.components.my-component.hbs

<br><br>

<div id="a-nice-person">
  Nice person
</div>
<div class="ember-view">
  <!-- rest of your Ember app's DOM... -->
</div>

{{#if isShowing}}
  {{#ember-tether
      target='#a-nice-person'
      targetAttachment='top left'
      attachment='bottom left'
  }}
   <div id="my-puppy">
    A puppy
   </div>
  {{/ember-tether}}
{{/if}}

<button onclick={{action 'clicked'}}>Button</button>

tests.helpers.destroy-app.js

import Ember from 'ember';

export default function destroyApp(application) {
  Ember.run(application, 'destroy');
}

tests.helpers.resolver.js

import Resolver from '../../resolver';
import config from '../../config/environment';

const resolver = Resolver.create();

resolver.namespace = {
  modulePrefix: config.modulePrefix,
  podModulePrefix: config.podModulePrefix
};

export default resolver;

tests.helpers.start-app.js

import Ember from 'ember';
import Application from '../../app';
import config from '../../config/environment';

const { run } = Ember;
const assign = Ember.assign || Ember.merge;

export default function startApp(attrs) {
  let application;

  let attributes = assign({rootElement: "#test-root"}, config.APP);
  attributes = assign(attributes, attrs); // use defaults, but you can override;

  run(() => {
    application = Application.create(attributes);
    application.setupForTesting();
    application.injectTestHelpers();
  });

  return application;
}

tests.integration.components.my-component-test.js

import { moduleForComponent, test } from 'ember-qunit';
import hbs from 'htmlbars-inline-precompile';

moduleForComponent('my-component', 'TODO: put something here', {
  integration: true
});

test('it renders', function(assert) {

  // Template block usage:
  this.render(hbs`
    {{#my-component}}
      template block text
    {{/my-component}}
  `);

  assert.equal(this.$('#a-nice-person').text().trim(), 'Nice person');
  assert.equal(Ember.$('#my-puppy').length, 0);
  
  this.$('button').click();
  assert.equal(Ember.$('#my-puppy').text().trim(), 'A puppy');
  
  this.$('button').click();
  assert.equal(Ember.$('#my-puppy').length, 0);
});

tests.test-helper.js

import resolver from './helpers/resolver';
import {
  setResolver
} from 'ember-qunit';

setResolver(resolver);

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": true
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-tether": "0.4.1"
  }
}