It seems JFR is making the incorrect assumption that SystemDictionary::do_unloading() is only ever called in safepoints, and fails with:

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (open/src/hotspot/share/jfr/jni/jfrGetAllEventClasses.cpp:50), pid=13170, tid=13188
# assert(SafepointSynchronize::is_at_safepoint()) failed: invariant
#
# JRE version: Java(TM) SE Runtime Environment (14.0) (fastdebug build 14-internal+0-2019-11-18-0707099.pliden...)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 14-internal+0-2019-11-18-0707099.pliden..., mixed mode, tiered, z gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0xdd2ae8] JfrEventClasses::increment_unloaded_event_class()+0x48
#

Current thread (0x00007f57740646a0): ConcurrentGCThread "ZDriver" [stack: 0x00007f57780ca000,0x00007f57781ca000] [id=13188]

Stack: [0x00007f57780ca000,0x00007f57781ca000], sp=0x00007f57781c8680, free space=1017k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0xdd2ae8] JfrEventClasses::increment_unloaded_event_class()+0x48
V [libjvm.so+0xe4dd25] do_unloaded_klass(Klass*)+0xc5
V [libjvm.so+0x92a426] ClassLoaderData::classes_do(void (*)(Klass*))+0x36
V [libjvm.so+0x936bb7] ClassLoaderDataGraph::classes_unloading_do(void (*)(Klass*))+0x57
V [libjvm.so+0xe5379c] JfrTypeSet::serialize(JfrCheckpointWriter*, JfrCheckpointWriter*, bool, bool)+0x1a6c
V [libjvm.so+0xdb1e4d] JfrCheckpointManager::write_type_set_for_unloaded_classes()+0x6d
V [libjvm.so+0x160b4c6] SystemDictionary::do_unloading(GCTimer*)+0x216
V [libjvm.so+0x181cbad] ZUnload::unlink()+0x6d
V [libjvm.so+0x17cfa28] ZHeap::process_non_strong_references()+0x38
V [libjvm.so+0x17cc4f0] ZDriver::gc(GCCause::Cause)+0x700
V [libjvm.so+0x17cd2fb] ZDriver::run_service()+0xcb
V [libjvm.so+0x9ccddb] ConcurrentGCThread::run()+0x2b
V [libjvm.so+0x16775c6] Thread::call_run()+0xf6
V [libjvm.so+0x13a954e] thread_native_entry(Thread*)+0x10e

To reproduce (with a debug build):

make test TEST=test/jdk/jdk/jfr/jvm/TestUnloadEventClassCount.java JTREG="VM_OPTIONS=-XX:+UnlockExperimentalVMOptions -XX:+UseZGC"