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
Point hoverEventLocation= textViewerExtension2.getHoverEventLocation();
			int offset= computeOffsetAtLocation(sourceViewer, hoverEventLocation.x, hoverEventLocation.y);
			if (offset == -1) {
				fTextOperationAction.run();
				return;				
			}				
			try {
				
				String contentType= TextUtilities.getContentType(sourceViewer.getDocument(), IJavaPartitions.JAVA_PARTITIONING, offset, true);
				IRegion hoverRegion= textHover.getHoverRegion(sourceViewer, offset);						
				if (hoverRegion == null)
					return;
				
				String hoverInfo= textHover.getHoverInfo(sourceViewer, hoverRegion);
				IInformationControlCreator controlCreator= null;				
				if (textHover instanceof IInformationProviderExtension2)
					controlCreator= ((IInformationProviderExtension2)textHover).getInformationPresenterControlCreator();
	
				IInformationProvider informationProvider= new InformationProvider(hoverRegion, hoverInfo, controlCreator);
				fInformationPresenter.setOffset(offset);	
				fInformationPresenter.setDocumentPartitioning(IJavaPartitions.JAVA_PARTITIONING);
				fInformationPresenter.setInformationProvider(informationProvider, contentType);
				fInformationPresenter.showInformation();
			} catch (BadLocationException e) {				
			}
		}
		
		private int computeOffsetAtLocation(ITextViewer textViewer, int x, int y) {
			
			StyledText styledText= textViewer.getTextWidget();