Fix eventgroup.

src/mtime_t_event.inc

@@ -127,9 +127,10 @@
     TYPE(t_event), TARGET :: event_to_add
     IF (.NOT. ASSOCIATED(this%last_event_in_group)) THEN
       this%first_event_in_group => event_to_add
-      this%first_event_in_group%nextEventInGroup => NULL()
+      NULLIFY(this%first_event_in_group%nextEventInGroup)
     ELSE
       this%last_event_in_group%nextEventInGroup => event_to_add
+      NULLIFY(event_to_add%nextEventInGroup)
     ENDIF
     this%last_event_in_group => event_to_add
   END SUBROUTINE t_eventGroup_addToGroup