Fix crash when connecting to a Bluetooth device

[Redirion]

What is it?

• Bugfix (user facing)
• Feature (user facing)
• Codebase improvement (dev facing)
• Meta improvement to the project (dev facing)

Description of the changes in your PR

The last weeks and NewPipe versions I had the issue that as soon as my phone connected via BT with my car NewPipe crashed as for some reason "onCompleted" was called.

`

java.lang.NullPointerException: Attempt to invoke virtual method 'int org.schabi.newpipe.player.playqueue.PlayQueue.getIndex()' on a null object reference
org.schabi.newpipe.player.Player.onCompleted
org.schabi.newpipe.player.Player.changeState
org.schabi.newpipe.player.Player.onPlayerStateChanged

Maybe I am the only one and this is a specific issue with my current car / car software, but as we have the same null checks in other listener methods, I felt it doesn't hurt to add it here as well.

Fixes the following issue(s)

#6233

APK testing

On the website the APK can be found by going to the "Checks" tab below the title and then on "artifacts" on the right.

Due diligence

• I read the contribution guidelines.

[triallax]

I can't seem to reproduce the NPE with this. Code also LGTM.