Is fearing that a calamity will occur considered a bad assumption about Allah? And does fearing a calamity lead to its occurrence? And is fear incompatible with having a good assumption about Allah?

Fear of an anticipated misfortune is a natural fear and is not considered a bad opinion of Allah. It is an innate fear, even for prophets, and the Prophet, peace be upon him, used to seek refuge from misfortunes. However, if this fear is based on pessimism, then it is considered a bad opinion of Allah. And if the fear exceeds what is natural and dominates one's thoughts, then it is from the whispers of Satan. As for the saying that fear of misfortune leads to its occurrence, it is incorrect unless the fear is based on pessimism and a bad opinion of Allah, for Allah may punish the pessimistic person by causing what he is pessimistic about to occur. For 'At-Tiyarah' (pessimism) exposes its adherent to evil and affliction.