intermediateTransitionTo()

application.controller.js

import Ember from 'ember';

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

application.route.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    // We simulate a model that would take 5 seconds to be loaded
    return new Promise((resolve, reject) => {
      Ember.run.later(() => {resolve();}, 5000);
    });
  },
  actions: {
    loading(transition, originRoute) {
      // Called while the model loads, but throws an exception
      this.intermediateTransitionTo('loading', {});
      // This would not throw an exception: this.intermediateTransitionTo('loading');
    }
  }
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

twiddle.json

{
  "version": "0.5.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}