Close icon is not displayed on new tab

[brichet]

Describe the bug
The close icon do not appear when adding a new closable tab in the main area (TabPanel).

To Reproduce
Create an extension which add a new widget in the main area, something like :

widget = new MainAreaWidget({content});
widget.id = 'new_widget';
widget.title.label = 'Courses';
widget.title.closable = true;

let w = app.shell.widgets('main').next() as TabPanel;
w.addWidget(widget);

Expected behavior
Have a close symbol on the tab, like :

Screenshots
Currently the result is :