Does the interpretation of a dream as the occurrence of something undesirable by a dream interpreter who meets the legitimate Islamic conditions necessarily mean that this undesirable thing will occur, or can taking precautionary measures to avoid it prevent its occurrence?

There is no impediment to taking precautions and exercising caution to prevent the occurrence of what was warned against in the dream, for its happening is not certain, and the interpreter may be right or may be wrong. Therefore, rely on Allah, have a good opinion of Him, and increase in supplication.