Paper chips component throws an 'focus' error with readOnly property as true

The issue can be reproduced on the demo page containing paper-chips examples.

When we change the `readOnly` property to be true and click into the paper-chips container, an error is thrown:
```
Uncaught TypeError: Cannot read property 'focus' of null
```
In my application, we still using it `ember-paper 1.0.0-beta-24` version.

I'm submitting this issue because I did notice that the error was thrown on the ember-paper examples as well.

**browser:** Google chrome (v 86.0.4240.183)