xquery:fork-join vs. WeakTokenSet?

[ChristianGruen]

The WeakTokenSet class may return null references:

[xquery:error] Unexpected error: java.lang.NullPointerException:
Cannot invoke "org.basex.util.hash.WeakTokenSet$WeakTokenRef.get()"
  because "this.keys[i]" is null

It is unclear how to reproduce this. It could be caused by xquery:fork-join calls, which run multiple XQuery functions at the same time.

Introduced with the switch to JDK 17.