Map editor bug.

Perhaps your version of "Heroes" is not entirely complete: in repacks, their creators may have removed files and elements that they deemed unnecessary to reduce the size. Some removed individual objects from those not used in the game, but they were sometimes used by the authors of some maps (for example, one obelisk, a couple of witch huts).
In this case, a complete version of the game would help you. Save your/needed files from the game folders and install another version.