#8 <signal handler called>
#9 0x00007f95c078da48 in GrowableArrayBase::clear (this=0x0) at /home/rehn/source/jdk/ongit/dev-jdk/open/src/hotspot/share/utilities/growableArray.hpp:97
#10 TimePartitions::clear (this=0x7f95c1aeddb8 <ZStatPhase::_timer+56>) at /home/rehn/source/jdk/ongit/dev-jdk/open/src/hotspot/share/gc/shared/gcTimer.cpp:126
#11 GCTimer::register_gc_start (this=this@entry=0x7f95c1aedd80 <ZStatPhase::_timer>, time=...) at /home/rehn/source/jdk/ongit/dev-jdk/open/src/hotspot/share/gc/shared/gcTimer.cpp:33
#12 0x00007f95c151ed3c in ZStatPhaseCycle::register_start (this=<optimized out>, start=...) at /home/rehn/source/jdk/ongit/dev-jdk/open/src/hotspot/share/gc/z/zStat.cpp:634
#13 0x00007f95c14b9dc8 in ZStatTimer::ZStatTimer (phase=..., this=0x7f95bc1bfcd0) at /home/rehn/source/jdk/ongit/dev-jdk/open/src/hotspot/share/gc/z/zStat.hpp:309
#14 ZStatTimer::ZStatTimer (phase=..., this=0x7f95bc1bfcd0) at /home/rehn/source/jdk/ongit/dev-jdk/open/src/hotspot/share/gc/z/zStat.hpp:304
#15 ZDriverGCScope::ZDriverGCScope (request=..., this=0x7f95bc1bfca0) at /home/rehn/source/jdk/ongit/dev-jdk/open/src/hotspot/share/gc/z/zDriver.cpp:407
#16 ZDriver::gc (this=this@entry=0x7f95b805dd70, request=...) at /home/rehn/source/jdk/ongit/dev-jdk/open/src/hotspot/share/gc/z/zDriver.cpp:446
#17 0x00007f95c14bbc8f in ZDriver::run_service (this=0x7f95b805dd70) at /home/rehn/source/jdk/ongit/dev-jdk/open/src/hotspot/share/gc/z/zDriver.cpp:494
#18 0x00007f95c04cb84b in ConcurrentGCThread::run (this=0x7f95b805dd70) at /home/rehn/source/jdk/ongit/dev-jdk/open/src/hotspot/share/gc/shared/concurrentGCThread.cpp:51

Null here:
(gdb) p _phases
$1 = (GrowableArray<GCPhase> *) 0x0

   125 void TimePartitions::clear() {
   126 _phases->clear();

Test was executed with:
make test-vmTestbase_nsk_jdi CONF=fast JTREG="RETAIN=all -javaoptions:\"-XX:+SuppressFatalErrorMessage -XX:+UseZGC -XX:ZFragmentationLimit=0 -XX:ZCollectionInterval=0.01\""