Green Screen when open world

I have 1 world that i build almost 1 year 8 days...Then yesterday when i open the world i can play in 15-20 second then the green screen pop up.I try downgrade to 0.12.2,0.12.1 but it also same.I try used backup world but it also same.Then last way i reboot my device i download back and i try open back...the green screen popup still pop up...WHY THE BUG NEVER BEEN FIXED???