Notes do not work with this plugin (on some browsers)

[DenizThatMenace]

When enabling this plugin and loading slides from external files the text-content appears just fine.

However, the speaker-notes that were added using the <aside class="notes">...</aside> syntax do not appear in the speaker's window.

Is this a known limitation of external?
Is that a fixable bug?

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I tried it both with external's async option enabled or disabled.
When enabled the notes are not shown but the Javascript-console of my browser (Chromium v69.0.3497.81) does not show any errors. When disabled, it shows some errors from the notes.js plugin-script:

Uncaught TypeError: Cannot read property 'querySelector' of undefined
    at post (notes.js:64)
    at onConnected (notes.js:123)
    at notes.js:53

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I also tried it out with some other browsers (all on my Ubuntu 16.04 Linux machine). These are the total results:

• Chromium (69.0.3497.81): Only text.content works (regardless of the value for option async).
• Firefox (v62.0.3): Everything works as long as I am not enabling asnyc option. If async option is enabled not even the text-content is displayed.
• Chrome (v69.0.3497.100): Everything works (regardless of the value for option async).

(BTW: I am using a local http-server to load the slides from a browser. Otherwise I would not be able to display anything due to (most) browsers' security-restrictions.)

[DenizThatMenace]

Some additional observations:

• In Chrome the notes are displayed in the speaker's windows, however, the timer is not running...
• In Firefox everything seems to work, when enabling the async option "externally". (See my comment at issue Any interest in a async version? #13.)

It looks to me as if it were some kind of race-condition between the individual (async) plugins.