[BUGFIX beta] Fix issues with revalidation during teardown.

[rwjblue]

When the toplevelView (aka ownerView) is being destroyed (during test teardown generally) an error was ocurring under certain "special" circumstances. Specifically, if removal of a sibling node triggered a revalidation of one still attached.

Consider the following example:

{{#x-select as |select|}}
 {{#select.option value="1"}}{/select.option}}
 {{#select.option value="2"}}{/select.option}}
{{/x-select}}

The components in question are using the registration pattern so that the children notify/register with the parent upon didInsertElement (and willDestroyElement).

When a new option is added or removed from the parent, it updates its value property which is bound to each options selected attribute (to toggle the selection state in the DOM).

The specific mechanism that causes the DOM to get updated when the various computed properties change is that a Stream object calls ownerNode.emberView.scheduleRevalidate. This tells the rest of the system to start walking the tree to flush out any updates to the DOM.

When a view is being removed, we set the emberView property of its renderNode to null before traversing the subtree of children. This means, that as the children are removed (and therefore cause the value of the parent to change) the selected attribute binding attempts to call ownerNode.emberView.scheduleRevalidation().

Unfortunately, when we are actually tearing down the ownerNode.emberView itself that invocation results in an error ( cannot call scheduleRevalidation on null).

---

This change adds a simple guard to avoid calling scheduleRevalidation when the ownerNode.emberView is being torn down.

Fixes #13846.
Fixes #13968.

[rwjblue]

I have uploaded the assets to make it a bit easier to test this in an ember-cli app. All you should need to do is change your bower.json from:

    "ember": "~2.8.0-beta.1",

To:

    "ember": "rwjblue/ember#pr-14110",

[rwjblue]

Since @krisselden worked on #13775, I'd love his review here...

[cibernox]

I confirm that this branch fixes the test suite for Ember Power Select: https://travis-ci.org/cibernox/ember-power-select/jobs/154144524#L332

[backspace]

Thanks so much for this fix!

[cibernox]

@rwjblue Could you port this commit to the beta branch so ember-try pulls the fix and addons got pretty green travis badges again 😄