The color pickers from Theme customization are not always loading

This is an older issue and hard to reproduce. I didn't find a pattern so far for consistent reproduction (but it reproduced on all supported browsers on my side), and was also reported by some user on the forum a time ago.

Steps to reproduce

1. Go to Administer Wiki > Look & Feel > Themes
2. Click 'Customize' button corresponding to the current set color theme
3. From the left, click on a tab which requires the color pickers (e.g. 'Base Colors')
4. Observe the color pickers

Expected results

The color pickers are always loaded.

Actual results

Sometimes, the color pickers are not loading, even after some page refreshes (or hard refreshes).

Usually there are no errors in JS console or in XWiki logs, but I managed to get an error in JS console this time on XWiki 16.10.6:

Failed to load resource: the server responded with a status of 404 () :1115/xwiki/bin/skin/skins/flamingo/resources/js/scriptaculous/scriptaculous.js:1

Another time while reproducing the issue, there was another error displayed in JS console, but I don't know for sure if it's related (see the attached screenshots).

Eventually, after some more page refreshes, the pickers are loading.