For info FullCalendar V6 use Preact internally.
Preact is similar to OWL/React/Vue as when some props change it apply
the change on the element linked to the props.
In our case for the events, when an event has a duration of 15 minutes
FullCalendar changes the `isShort` to `true` so Preact can add the
`fc-timegrid-event-short` class to the event.

In Odoo we add also other classes to these events for our needs
(e.g.: `o_event_striked`).
In FullCalendar V4 it was done in `eventRender` using
`el.classList.add()`. During the migration to FullCalendar V6 [1]
the `eventRender` was changed into `eventDidMount` (a Preact HOOK).

In most cases, `eventDidMount` is fine but not for short event, as
in FullCalendar V6 there is another HOOK especially for the classes
`eventClassNames` [2].

Before this fix, the render flow of the event was:
1) pre-render event
2) add the event to the DOM
3) call `eventDidMount` (here we add our classes)
4) FullCalendar set `true` to `isShort` props
5) Preact sets the classes on the event to add `fc-timegrid-event-short`
   (here FullCalendar removes our classes added in `eventDidMount`)

After the fix, the render flow is the same as before, but as we add the
classes using the `eventClassNames` HOOK, FullCalendar knows all
additional classes per event so when Preact (in steps 5) sets the
classes its adds `fc-timegrid-event-short` and all other classes added
by our code in `eventClassNames`.

Steps to reproduce:
* Open Calendar App
* Make an event with a duration of 15 minutes
* Refresh the page (F5)
  => Bug the style of new event (15 min) is wrong

[1]: odoo/odoo@90f85a19deaea33cd747c969762ff20f1d59ef4c
[2]: https://fullcalendar.io/docs/event-render-hooks

---
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Currently, an error is generated when the user removes the parent category from the
POS product categories and tries to save it.

Steps to produce and error:
1. Go to Point of Sale > Configuration > Products > PoS Product Categories
2. Open any PoS product categories that contains Parent Category.
3. Now remove Parent Category and try to save it >>> error generate

Stack Trace:
```
IndexError: list index out of range
  File "odoo/http.py", line 2251, in __call__
    response = request._serve_db()
  File "odoo/http.py", line 1827, in _serve_db
    return self._transactioning(_serve_ir_http, readonly=ro)
  File "odoo/http.py", line 1847, in _transactioning
    return service_model.retrying(func, env=self.env)
  File "odoo/service/model.py", line 134, in retrying
    result = func()
  File "odoo/http.py", line 1825, in _serve_ir_http
    return self._serve_ir_http(rule, args)
  File "odoo/http.py", line 1832, in _serve_ir_http
    response = self.dispatcher.dispatch(rule.endpoint, args)
  File "odoo/http.py", line 2057, in dispatch
    result = self.request.registry['ir.http']._dispatch(endpoint)
  File "odoo/addons/base/models/ir_http.py", line 220, in _dispatch
    result = endpoint(**request.params)
  File "odoo/http.py", line 739, in route_wrapper
    result = endpoint(self, *args, **params_ok)
  File "addons/web/controllers/dataset.py", line 38, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "addons/web/controllers/dataset.py", line 34, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "odoo/api.py", line 458, in call_kw
    result = getattr(recs, name)(*args, **kwargs)
  File "addons/web/models/models.py", line 71, in web_save
    self.write(vals)
  File "addons/point_of_sale/models/pos_category.py", line 67, in write
    vals["color"] = self.search_read([("id", "=", vals["parent_id"])])[0][
```

This is because at [1], we receive the `False` value in `vals["parent_id"]` as a result of
the search; it has no value, and a key error is generated because we try to access the first element but it is empty.

This commit will fix this issue by ensuring that the value of `parent_id` is in `vals` before searching.

[1]-https://github.com/odoo/odoo/blob/7479ffdc43c857bfd6439aefc024330dc22825b3/addons/point_of_sale/models/pos_category.py#L66-L69

sentry-5153069546