This bug is fixed on beta but not yet made public. On beta I can't post online and the whole reason I need to sign in is to play online. 

When I tap sign in, it says signing in with your Microsoft account. then says "Minecraft stopped working". I've signed in many times before and it works fine.

I've tried signing in from settings but it still doesn't work. I've reinstalled it, cleared cache but it never works. Like I said it is fixed on beta so please update it very soon.