Bug 75711 AstProvider should not create AST if requested by SelectionListenerWithASTManager
3.1 M2 If SelectionListenerWithASTManager waits for an AST and the editor closes we should not create an AST for that one but return null instead.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/JavaEditor.java
return;
			}
		
			fURLString= null;
			IRegion region= getCurrentTextRegion(viewer);
			if (region == null || region.getLength() == 0) {
				repairRepresentation();
				return;
			}
			
			highlightRegion(viewer, region);
			activateCursor(viewer);												
		}
		/*
		 * @see org.eclipse.swt.events.FocusListener#focusGained(org.eclipse.swt.events.FocusEvent)
		 */
		public void focusGained(FocusEvent e) {}
		/*
		 * @see org.eclipse.swt.events.FocusListener#focusLost(org.eclipse.swt.events.FocusEvent)
		 */
		public void focusLost(FocusEvent event) {
			deactivate();
		}
		/*
		 * @see org.eclipse.jface.text.IDocumentListener#documentAboutToBeChanged(org.eclipse.jface.text.DocumentEvent)
		 */
		public void documentAboutToBeChanged(DocumentEvent event) {
			if (fActive && fActiveRegion != null) {
				fRememberedPosition= new Position(fActiveRegion.getOffset(), fActiveRegion.getLength());
				try {