[fix practice_add_new_report] Add a condition to check if the practice supervisor is selected.

[true-real-michael]

Description

If a new report is added while the practice supervisor is not selected, the application crashes with 500: Internal Server Error.

This PR fixes this error and suggests the user to select the supervisor via a flash error message.

Steps to reproduce the bug

1. Go to https://se.math.spbu.ru/practice or a locally hosted alternative
2. Add a new practice via the button "Новая работа".
3. Do NOT select a supervisor (note that the service allows to create a new practice without a name and without supervisor selected).
4. In the created practice go to "Отчётность".
5. Add a new report

Context

This line tries to find a supervisor but fails because current_thesis.supervisor_id is None

spbu_se_site/src/flask_se_practice.py

Line 429 in 2cd65d2

Staff.query.filter_by(id=current_thesis.supervisor_id).first().user_id

Suggested solution

A condition is added to check if the supervisor is selected. In case if the supervisor is not selected flash an error message which suggests to select the supervisor.

Alternative solution

Prohibit to create a new practice unless a supervisor is selected.

[iakov]

Nice PR, thanks. I've fixed formatting, and it is fine to be merged.