public void onClose() {
		GFileSystem fsCopy;
		synchronized (this) {
			if (fs == null) {
				throw new IllegalArgumentException("FileSystemRefManager already closed!");
			}
			if (!refs.isEmpty()) {
				Msg.warn(this, "Closing filesystem even though it has active handles open: " + fs);
			}
			fsCopy = fs;
			fs = null;
			refs.clear();
			refs = null;
		}
		listeners.invoke().onFilesystemClose(fsCopy);
	}
Called from the {@link GFileSystem#close()} before any destructive changes have
been made to gracefully shutdown the ref manager.
<p>
Broadcasts {@link FileSystemEventListener#onFilesystemClose(GFileSystem)}.
Ghidra/Features/Base/src/main/java/ghidra/formats/gfilesystem/FileSystemRefManager.java