java.lang.NullPointerException: Ticking player

I don't know if this duplicates or relates to MC-103601, I searched for "ticking player" on Mojira's open bugposts, but couldn't find the same, only other bugposts containing "ticking", sorry if I overlooked an open bugpost for "player".

I did find a closed/fixed bugpost for "ticking player" from 2013 though: MC-35653

How it happened was: I opened up one of my bugtest worlds in 17w17a and upon entering immediately crashed with that error message in my client output; crash log file attached.

When I entered the world again (in 17w17a), everything was fine though, I didn't crash again.

I'll see if I can maybe reproduce it again, until then I'll leave out the "b"-version as affected, as it happened in "a". Maybe the crashlog can provide some information what was happening.