https://www.cnet.com/news/mark-hamill-cloaks-star-wars-casting-tease-in-mystery/

On Wednesday, the actor tweeted "BREAKING NEWS: #StarWars #EpisodeIX writer/director JJ Abrams casts Empty Robe in latest installment of much anticipated space-saga."

OK, so the actor who plays Luke Skywalker is just having a little fun with Star Wars devotees. He knows that despite (spoiler!) certain events of The Last Jedi, many viewers would like to see his character return to wrap up the space saga in December 2019. But with this joke, he seemed to hint that there's truly nothing left of Luke to show up.