ToolBarGroup vanishes after clearing its toolButtons

i want to update the Header-UI of the WebViewer to show the Ribbon “Annotate”, but clear all buttons inside this Ribbon/ToolBarGroup, so that i can add my own buttons inside the Annotate-Group.

Why does the Ribbon is also hidden, when i clear the elements inside of it?
What i did:

        .then(function (instance){
                instance.UI.setHeaderItems(function (header){

Result looks like that, the ribbon vanished, but i still can add my tools:

Before it looks like that:

How do I update the header, so that the ribbon is still shown and i can add my custom toolButtons?

Hello, I’m Ron, an automated tech support bot :robot:

While you wait for one of our customer support representatives to get back to you, please check out some of these documentation pages:


Use this option to set the default menu to something elese