Bug 147377 [Field Assist] - open proposal popup should close when there are no valid proposals left
bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/ContentProposalAdapter.java
/*
	 * Empty string.
	 */
	private static final String EMPTY = ""; 
	/*
	 * The object that provides content proposals.
	 */
	private IContentProposalProvider proposalProvider;
	/*
	 * A label provider used to display proposals in the popup, and to extract
	 * Strings from non-String proposals.
	 */
	private ILabelProvider labelProvider;
	/*
	 * The control for which content proposals are provided.
	 */
	private Control control;
	/*
	 * The adapter used to extract the String contents from an arbitrary
	 * control.
	 */
	private IControlContentAdapter controlContentAdapter;
	/*
	 * The popup used to show proposals.
	 */
	private ContentProposalPopup popup;
	/*
	 * The keystroke that signifies content proposals should be shown.
	 */
	private KeyStroke triggerKeyStroke;