Bug 277966 [Databinding] regression: ObservablesManager throws NPE on dispose if one of the observables is already disposed
bundles/org.eclipse.core.databinding/src/org/eclipse/core/databinding/ObservablesManager.java
private Set excludedObservables = new HashSet();
	private Map contexts = new HashMap();
	/**
	 * Create a new observables manager.
	 */
	public ObservablesManager() {
	}
	/**
	 * Adds the given observable to this manager.
	 *
	 * @param observable
	 *            the observable
	 */
	public void addObservable(IObservable observable) {
		managedObservables.add(observable);
	}
	/**
	 * Adds the given observable to this manager's exclusion list. The given
	 * observable will not be disposed of by this manager.
	 *
	 * @param observable
	 *            the observable
	 */
	public void excludeObservable(IObservable observable) {
		excludedObservables.add(observable);
	}
	/**
	 * Adds the given data binding context's target and/or model observables to
	 * this manager.
	 *