When launching Theme Builder on a subsite, you might come across an error when you see the loading screen stuck like in the screenshot below, and the Theme Builder does not start.
This issue happens with ShortPoint SPFx while installing the app on Office 365 (SharePoint Online) environment.
In the case described above, please check if ShortPoint is installed on the upper-level sites of the site collection. If the error appears on a first-level subsite, you need to install ShortPoint in the root site of the site collection. If this is a second-, third- or greater-level subsite, ShortPoint needs to be installed in all upper level subsites of the site collection, up to the root site of the site collection, including the root site of the site collection.
Below is a diagram to illustrate an example.
If the Theme Builder error is discovered on Subsite BA, you need to add ShortPoint to the top level site, Subsite B, as well as to the Root Site, for the issue to be fixed. The app can afterwards be safely removed from the Subsite B and Root Site.
Here is the only step that you need to do on the top level sites:
These are temporary measures. After ShortPoint is installed on these sites, you can safely remove the app from these sites.
You can as well leave the app in these sites if you prefer.
This will solve the issue and the Theme Builder will launch normally in the subsite.