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
length= widgetRange.getLength();
				
			} else {
				offset= region.getOffset() - viewer.getVisibleRegion().getOffset();
				length= region.getLength();
			}
			text.redrawRange(offset, length, false);
			
			
			fActiveRegion= region;
			if (viewer instanceof ITextViewerExtension2)
				((ITextViewerExtension2) viewer).invalidateTextPresentation(region.getOffset(), region.getLength());
			else
				viewer.invalidateTextPresentation();
		}
		
		private void activateCursor(ISourceViewer viewer) {
			StyledText text= viewer.getTextWidget();
			if (text == null || text.isDisposed())
				return;
			Display display= text.getDisplay();
			if (fCursor == null)
				fCursor= new Cursor(display, SWT.CURSOR_HAND);
			text.setCursor(fCursor);
		}
		
		private void resetCursor(ISourceViewer viewer) {
			StyledText text= viewer.getTextWidget();
			if (text != null && !text.isDisposed())
				text.setCursor(null);