Which of the following is NOT a valid reason for an employer to terminate the employment contract without award, advance notice, or indemnity?

The worker has unlawfully taken advantage of his position for personal gain.

The worker assaults the employer, the manager in$-$charge, or any of his superiors during or by reason of the work.

The worker takes a sick leave for $15$ consecutive days due to a serious medical condition.

The worker deliberately commits or omits any act with the intent to cause material loss to the employer, provided that the employer reports the incident to the competent authorities within $24$ hours after becoming aware of its occurrence.