Notifications do not work without a template

[TheoMathurin]

Panel 0.13.1

As mentioned in #3819, notifications do not seem to work without a template. They do not work in a notebook either.

I've not seen anything that says this is expected behavior (they do not work in the dedicated docs notebook), so I report it as a bug.

import panel as pn
pn.extension(notifications=True)


def callback(event):
    pn.state.notifications.error('Error')


button = pn.widgets.Button(name='Display error')
button.on_click(callback)

without_template = True
if without_template:
    button.servable() # does not work
else:
    pn.template.FastListTemplate(main=[], sidebar=[button]).servable() # works fine