We have this error handling:

err:
/* Let any running threads finish up. */
if (multithreaded) {
__wt_cond_signal(session, conn->layered_drain_data.threads.wait_cond);
__wt_writelock(session, &conn->layered_drain_data.threads.lock);
WT_TRET(__wt_thread_group_destroy(session, &conn->layered_drain_data.threads));
}

We set bool multithreaded = conn->layered_drain_data.thread_count > 1; on line 2062, but do not create the thread groups until afterwards. If thread_group_create fails, it may be possible to have a NULL value for threads.wait_cond.
 
Dereference of potentially null field

If the field is actually null, then a null pointer dereference will occur. Field, which is statistically checked against null, is dereferenced.
/src/conn/conn_layered.c:2111: NULL_FIELD 184202 Reading field "wait_cond", which is expected to possibly be "NULL" in "conn->layered_drain_data.threads.wait_cond" (checked 1 out of 13 times).