Not likely to be an issue for you. But if the only use in the movie is as wardrobe for a character with no connection to Disney, it shouldn't be an issue. Removing the buttons was a good idea and should demonstrate your good faith in avoiding any connection with Disney (unless someone takes the cock-eyed view that you're removing copyright and trademark notices!). The only other potential for problem would be if you had entered into some license agreement, for example, if you bought directly from Disney and they made you use a click-through agreement that prohibits displaying the dress in commercial motion pictures ... a legal theory which sounds remarkably far-fetched as we write it.
Also, we're confused. We've been googling Princess dress ads (please Google, don't track that in our search history) and we can't tell for sure whether "Princess" refers to a line/brand of Disney dresses or to a specific princess/character? For example, the dress shown above is billed as "Disney Princess Role-Play Dress" but it has a picture of Sleeping Beauty on the tag, (so what is the role being "played"?). If the reference is to a line of dresses, the Dear Rich Staff believes that wearing one is probably even less of an issue for your movie.