Issue

When accessing Theme Builder from a Modern SharePoint subsite on Office 365, it is in the loading state indefinitely.  It does not proceed to show options to choose between modern and classic and customize your site.

Reason

You have not upgraded to ShortPoint SPFx version 6.8.19.0 through the App Catalog Upgrade (not through One-Click Upgrade).

Solution

First, Verify ShortPoint SPFx Version

Go to Site Contents of the subsite, click on show actions ellipsis beside the ShortPoint SPFx app, and choose Details in the drop-down. 

If you see version 6.7.128.0 or lower, you need to perform the steps described below to resolve the issue.  If you have version 6.8.19.0, please contact us at support@shortpoint.com.

Note: In the present case, please do not check the ShortPoint SPFx version by going to the Dashboard, as it will show the One-Click upgrade version, but will not show the latest App Catalog upgrade version.

Next, Perform A Full App Catalog Upgrade of ShortPoint

You can follow the steps shown in these support articles to perform a full app catalog upgrade with version 6.8.19.0.

Step 1: Upload new ShortPoint SPFx to Site Collection App Catalog (Office 365)

Step 2: Update ShortPoint SPFx on SharePoint Site (Office 365)

Step 3: Run ReInstallation and Upgrade from ShortPoint SPFx Dashboard (Office 365)

Once you have upgraded to ShortPoint SPFx version 6.8.19.0, Theme Builder will be accessible to edit your Modern subsite.