NullPointerException: job services

[ChristianGruen]

A registered job that is called in intervals may lead to a NullPointerException:

basex/basex-core/src/main/java/org/basex/core/jobs/QueryJob.java

Line 210 in 1b7a673

result.value = qp.value().materialize(d -> d == null || d.inMemory(), null, qp.qc);

java.lang.NullPointerException: Cannot read field "qc" because "this.qp" is null
    at org.basex.core.jobs.QueryJob.run(QueryJob.java:210)
    at java.base/java.lang.Thread.run(Thread.java:1583)
java.lang.NullPointerException: Cannot read field "qc" because "this.qp" is null
    at org.basex.core.jobs.QueryJob.run(QueryJob.java:210)
    at java.base/java.lang.Thread.run(Thread.java:1583)

The number of NPEs pile up when the requests runs for a longer time.