No one can objectively say what a dream means, but it's at least useful to consider what unprocessed emotions you have about your ex that may have caused the dream.

Consider this: what if you saw your ex walking down the street toward you in some random part of the country where you would least suspect it?

What issues come up for you in that moment, if any?

What would you be avoiding in that interaction?

What would your needs be in that interaction?

Is there anything left unsaid between you two? Even if it's a simple "thank you"?