Refreshing playground page can sometimes break the editor pane (editor goes blank)

## 🐛 Bug Report

### Current behavior

The playground's editor pane can become completely blank by refreshing the playground page.

So far I was able to reproduce this in two ways:

### Repro 1

1. Open a new tab and navigate to https://playground.amp.dev
2. Refresh the page

### Repro 2

1. Open a new tab and navigate to https://playground.amp.dev
2. Paste in a playground share link into the browser address bar copied from the "Share" button.

In both cases, I can observe that the editor goes blank. However, there's still a very small-width column on the right edge of the editor that's still visible, especially when you vertically scroll in the editor.

See screenshot:

- What browser are you using?

Chrome

- What O/S are you using?

Mac OS

/to @sebastianbenz