This traceback arises when the user tries to attach a receipt in the expense

To reproduce this issue:

1) Install `hr_expense`
2) Open any existing `my expense` record
3) Click on the `Attach Receipt` button and attach a file 
4) Now again attach a file through the `Attach Receipt` button 
5) Now this time click on `cancel` while attaching a file through `Attach Receipt`

Error:- 
```
IndexError: list index out of range
  File "odoo/http.py", line 2157, in __call__
    response = request._serve_db()
  File "odoo/http.py", line 1732, in _serve_db
    return service_model.retrying(self._serve_ir_http, self.env)
  File "odoo/service/model.py", line 133, in retrying
    result = func()
  File "odoo/http.py", line 1759, in _serve_ir_http
    response = self.dispatcher.dispatch(rule.endpoint, args)
  File "odoo/http.py", line 1960, in dispatch
    result = self.request.registry['ir.http']._dispatch(endpoint)
  File "odoo/addons/base/models/ir_http.py", line 207, in _dispatch
    result = endpoint(**request.params)
  File "odoo/http.py", line 722, in route_wrapper
    result = endpoint(self, *args, **params_ok)
  File "addons/web/controllers/dataset.py", line 24, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "addons/web/controllers/dataset.py", line 20, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "odoo/api.py", line 466, in call_kw
    result = _call_kw_multi(method, model, args, kwargs)
  File "odoo/api.py", line 453, in _call_kw_multi
    result = method(recs, *args, **kwargs)
  File "addons/hr_expense/models/hr_expense.py", line 484, in attach_document
    self.message_main_attachment_id = kwargs['attachment_ids'][-1]
```

When the user clicks on the `cancel` button when trying to attach a file, 
the `kwargs` returns an empty `attchemnt_ids` record.

This leads to the above traceback from here

https://github.com/odoo/odoo/blob/5d00b4146aa5cc1942d66e60dc92d321f79bf891/addons/hr_expense/models/hr_expense.py#L482-L484

sentry-4705156379

In the `onMounted` hook of the calendar renderer, the given callback function will schedule a new function call using `setTimeout` with a delay of 0 if some conditions are satisfied. The scheduled function will then trigger a scroll in the calendar view to display a precise timeslot.

If the component is destroyed before the scheduled function is executed, the scheduled function may trigger an error as the root element of the calendar will be removed from the DOM when the component is destroyed.

This commit will fix that issue by checking that the root element of the calendar exists before calling the `scrollToTime` function.

Steps to reproduce the error:
1. Open Knowledge
2. Open the template gallery
3. Select the template "Sprint Calendar"
4. Quickly apply the template

=> If you have the right timing: a traceback will be displayed indicating that the function `scrollToTime` is not defined on `null`.

TO BE: No traceback should be displayed when applying the template.

task-3627780

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