What is the problem?

When trying to add the 51st favourite template, I see in the console:

Uncaught (in promise) undefined

Happens for VisualEditor and WikiEditor.

I don't see anything bad happen as a result of this.

Steps to reproduce problem

1. Login to https://en.wikipedia.beta.wmflabs.org
2. Go to https://en.wikipedia.beta.wmflabs.org/w/index.php?title=BeforeEach-name-0.4566901484662702-I%C3%B1t%C3%ABrn%C3%A2ti%C3%B4n%C3%A0liz%C3%A6ti%C3%B8n&action=edit
3. Add 50 favourite templates
4. Open the browser console
5. Try to add another template to your favourites
   1. Also repeat this in the VisualEditor

(Locally you could also set something like $wgTemplateDataMaxFavorites = 5 so you only need to add 5 favourite templates in step 3.)

Environment

Browser: Firefox 128. Chromium 135.
Wiki(s): https://en.wikipedia.beta.wmflabs.org TemplateData 0.2.0 (8bb5fb2) 07:29, 22 April 2025.
Editor: WikiEditor 0.5.4 (5c0d64c) 07:35, 21 April 2025. VisualEditor – (0c40a9a) 07:29, 22 April 2025.