browser example seems to be broken

Hi, I'm interested in integrating Gutenberg into one of my projects and I tried to get a sense of how the integration would look like by running the example in `src/browser/index.html`.

However I noticed that styling in this example seems to be broken and various actions in the editor result in different Javascript errors being logged to console.

For example, if I load up an empty editor, click +, and add "Columns", the styles are missing and Javascript errors are logged to console:


If I then click on the row below the columns, the + button appears on the left side instead of the right, which I think is another styling error. I checked out a few older releases and found that 2.23.1 mostly looks and works the way I expect it to, but 2.24.0 seems to be the first release that was "broken" in this way.

Is any of this expected behaviour or did something go wrong with recent releases?

Start with src/browser/index.html and reproduce the empty-editor Columns steps described in the issue, checking the browser console and the misplaced + button. Compare the behavior with releases 2.23.1 and 2.24.0 to narrow down when it changed. Done means the browser example has the expected styling and editor actions no longer produce the reported errors.

Written by the indexing model from the issue text.