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/search/FindOccurrencesEngine.java
protected IOccurrencesFinder getOccurrencesFinder() {
		return fFinder;
	}
	public String run(int offset, int length) throws JavaModelException {
		ISourceReference sr= getSourceReference();
		if (sr.getSourceRange() == null) {
			return SearchMessages.getString("FindOccurrencesEngine.noSource.text");  
		}
		
		final CompilationUnit root= createAST();
		if (root == null) {
			return SearchMessages.getString("FindOccurrencesEngine.cannotParse.text"); 
		}
		String message= fFinder.initialize(root, offset, length);
		if (message != null)
			return message;
		
		final IDocument document= new Document(getSourceReference().getSource());
		
		if (USE_NEW_SEARCH) {
			performNewSearch(fFinder, document, getInput());
			return null;
		}
		
		final IWorkspaceRunnable runnable= new IWorkspaceRunnable() {
			public void run(IProgressMonitor monitor) throws CoreException {
				ISearchResultView view= startSearch();
				fFinder.perform();
				IResource file= getMarkerOwner();				
				IMarker[] markers= fFinder.createMarkers(file, document);