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/SemanticHighlightingReconciler.java
for (int i= 0, n= subtrees.length; i < n; i++)
			subtrees[i].accept(fCollector);
		List oldPositions= fRemovedPositions;
		List newPositions= new ArrayList(fNOfRemovedPositions);
		for (int i= 0, n= oldPositions.size(); i < n; i ++) {
			Object current= oldPositions.get(i);
			if (current != null)
				newPositions.add(current);
		}
		fRemovedPositions= newPositions;
	}
	/**
	 * Update the presentation.
	 * 
	 * @param textPresentation the text presentation
	 * @param addedPositions the added positions
	 * @param removedPositions the removed positions
	 */
	private void updatePresentation(TextPresentation textPresentation, List addedPositions, List removedPositions) {
		Runnable runnable= fJobPresenter.createUpdateRunnable(textPresentation, addedPositions, removedPositions);
		if (runnable == null)
			return;
		
		JavaEditor editor= fEditor;
		if (editor == null)
			return;
		
		IWorkbenchPartSite site= editor.getSite();
		if (site == null)