According to Jim Henson’s Red Book, Jim (who served as executive producer on the film) was extremely busy in the months leading up to the 1990 premiere of The Witches, but he did get heavily involved in promoting the film.
As part of the promotion, Jim recorded a short video that gave a behind the scenes look at the special effects created by his Creature Shop.
The Witches was the final film that Jim personally worked on before his death.