List<MarkerSetImpl> copyList() {
			return new ArrayList<>(markerSets);
		}
Use this close listener approach instead of plugin events, since we don't get a
ProgramClosedPluginEvent when a trace view is closed, but we can listen for its
domain object closing, which works for plain programs, too.
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/marker/MarkerManager.java