Solved: How to add submenu to Tinymce v5.8 Splitbutton dropdown list

Using Tinymce v5.8 in a asp.net web application. I have added a splitbutton component to the toolbar and successfully added items to the splitbutton's dropdown list. Is there now a way to add a submenu to the list?

I have tried the attached code but the submenu does not appear. I guess that the "type: 'submenu' may be causing the issue but can find no documentation on this or could be that submenus simply are not a feature of this component. Any help or suggestions would be gratefully received.

  editor.ui.registry.addSplitButton('myButton', {
                     text: 'My Button',
                     icon: 'info',
                     tooltip: 'This is an example split-button',
                     onAction: function () {
                         //editor.insertContent('<p>You clicked the main button</p>');
                     },
                     onItemAction: function (api, value) {
                         editor.insertContent(value);
                     },
                     fetch: function (callback) {
                     
                         var items = [
                             { type: 'choiceitem', text: 'One', value: 'one ' },
                             { type: 'choiceitem', text: 'Two', value: 'two ' },
                             {
                              type: 'submenu',  text: 'Submenu', items: [
                                     { type: 'choiceitem', text: 'Three', value: 'three' }
                                 ]
                             }
                         ];

                         callback(items);
                     }
                 });

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