Cannot load the custom widget (kind: 'styling')

[#1]

I am following the sample "Colored Box" from the "SAP Analytics Cloud Custom Widget Developer Guide".

I am running a local server. Initially, 'main' and 'styling' could not be loaded. Then, I logged off from SAC and logged in again, but this time using a private browser. After this, 'main' could be loaded, but 'styling' could not.

I can see the red box (the custom widget works), but this message pops up:

Something went wrong. The system couldn't load the custom widget 'com.sap.sample.coloredbox_1.x' (kind: 'styling'). Please contact your system administrator or the owner of the custom widget.

Why does this issue happen?