Fix setEventNotificationMode for unmounted virtual threads

targetThread is NULL only for virtual threads, as per the assertion in
getVMThread, when mustBeAlive is TRUE. vmThreadForTLS is only used to
acquire J9JavaVM in createThreadData and jvmtiTLSGet. If targetThread
is NULL, currentThread is passed to createThreadData and jvmtiTLSGet
for retrieving J9JavaVM in JDK19+.

Related: eclipse-openj9#15541
Related: eclipse-openj9#15183

Signed-off-by: Babneet Singh <sbabneet@ca.ibm.com>

runtime/jvmti/jvmtiHelpers.c

@@ -1601,23 +1601,33 @@ setEventNotificationMode(J9JVMTIEnv * j9env, J9VMThread * currentThread, jint mo
 		eventMap = &(j9env->globalEventEnable);
 	} else {
 		j9object_t threadObject = J9_JNI_UNWRAP_REFERENCE(event_thread);
+		J9VMThread *vmThreadForTLS = NULL;
 		rc = getVMThread(currentThread, event_thread, &targetThread, TRUE, TRUE);
 		if (rc != JVMTI_ERROR_NONE) {
 			goto done;
 		}
+		vmThreadForTLS = targetThread;
 #if JAVA_SPEC_VERSION >= 19
 		rc = allocateTLS(vm, threadObject);
 		if (JVMTI_ERROR_NONE != rc) {
 			releaseVMThread(currentThread, targetThread, event_thread);
 			goto done;
 		}
+		if (NULL == targetThread) {
+			/* targetThread is NULL only for virtual threads, as per the assertion in getVMThread,
+			 * when mustBeAlive is TRUE. vmThreadForTLS is only used to acquire J9JavaVM in
+			 * createThreadData and jvmtiTLSGet. If targetThread is NULL, currentThread is passed
+			 * to createThreadData and jvmtiTLSGet for retrieving J9JavaVM in JDK19+.
+			 */
+			vmThreadForTLS = currentThread;
+		}
 #endif /* JAVA_SPEC_VERSION >= 19 */
-		rc = createThreadData(j9env, targetThread, threadObject);
+		rc = createThreadData(j9env, vmThreadForTLS, threadObject);
 		if (JVMTI_ERROR_NONE != rc) {
 			releaseVMThread(currentThread, targetThread, event_thread);
 			goto done;
 		}
-		eventMap = &(jvmtiTLSGet(targetThread, threadObject, j9env->tlsKey)->threadEventEnable);
+		eventMap = &(jvmtiTLSGet(vmThreadForTLS, threadObject, j9env->tlsKey)->threadEventEnable);
 	}
 
 	/* Single step can alloc/free the bytecode table, so we need exclusive to prevent any threads from using the table */