'App error: TypeError: dialog.showModal is not a function' occurs when trying the "Try it" link

[monetschemist]

In your README.md you have:

ezGantt uses public Google Chart and Google Drive JavaScript APIs, so the same users who can view the Spreadsheet can see the Gantt chart. To try it, go to https://ezgantt.appspot.com/#1kYfoEFXW-psZ8AMy0Ne7wRMiM3umNKBs3v9U7nvafXA

Clicking on this link this in Firefox 96 on Ubuntu 21.10 produces the noted error popup:

App error: TypeError: dialog.showModal is not a function

Clicking on the Edit link in the sample pops up the same error.

Meanwhile nothing much shows up in the screen itself beyond the header bar.