apiviewer hangs in some situation, showing the spinning wheel in the main window

[level420]

e.g when navigating to https://qooxdoo.org/qxl.apiviewer/#qx.ui.core.Widget~clone!method_public
the right region where the information is showing has obviously fully loaded, but remains covered by the grey blocker with the spinning wheel.

This may be due to the errors shown in console like this:

004286 qx.Promise: Unhandled promise rejection: TypeError: e.isPrivate is not a function
    at i.qxl.apiviewer.ui.ClassViewer.prototype.__enableSection() [as __enableSection] (https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:2681266)
    at i.qxl.apiviewer.ui.ClassViewer.prototype.showItem() [as showItem] (https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:2680528)
    at i.qxl.apiviewer.TabViewController.prototype.showItem() [as showItem] (https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:2632169)
    at https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:2613676
    at https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:2613134
From previous event:
    at T.B [as _captureStackTrace] (https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:297757)
    at T._then (https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:322696)
    at T.then (https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:321029)
    at i.qx.Promise.prototype._callMethod() [as _callMethod] (https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:278359)
    at i.qx.Promise.prototype.then() [as then] (https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:276590)
    at i.qxl.apiviewer.Controller.prototype.__selectItem() [as __selectItem] (https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:2613438)
    at https://qooxdoo.org/qxl.apiviewer/apiviewer/part-0.js:1:2610500