Bug 277966 [Databinding] regression: ObservablesManager throws NPE on dispose if one of the observables is already disposed
tests/org.eclipse.jface.tests.databinding/src/org/eclipse/core/tests/databinding/ObservablesManagerTest.java
IObservableValue {
		private boolean disposeCalled = false;
		/*
		 * (non-Javadoc)
		 *
		 * @see org.eclipse.core.databinding.observable.value.IObservableValue#addValueChangeListener(org.eclipse.core.databinding.observable.value.IValueChangeListener)
		 */
		public void addValueChangeListener(IValueChangeListener listener) {
			
		}
		/*
		 * (non-Javadoc)
		 *
		 * @see org.eclipse.core.databinding.observable.value.IObservableValue#getValue()
		 */
		public Object getValue() {
			
			return null;
		}
		/*
		 * (non-Javadoc)
		 *
		 * @see org.eclipse.core.databinding.observable.value.IObservableValue#getValueType()
		 */
		public Object getValueType() {