No, a person with PTSD, or who is under a lot of stress can experience nightmares about anything. Our brains try to process whatever we take in during the day. Our brains have a lot of information stored and as we sleep our brains try to put things together and store, so we can have stressful dreams that disturb our sleep/rest/restorative. As we make gains in "problem solving" we can add ways for our brains to utilize that while we sleep as well.