@Override
	void programClosed(Program closedProgram) {
		tree.cancelWork();

		closedProgram.removeListener(domainObjectListener);

		program = null;
		rebuildTree();

		closeComponent();
	}
A disconnected symbol tree is a snapshot of the primary symbol tree.
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/symboltree/DisconnectedSymbolTreeProvider.java