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
private IClassFile fClassFile;
		
		public FindOccurencesClassFileEngine(IClassFile file, IOccurrencesFinder finder) {
			super(finder);
			fClassFile= file;
		}
		protected CompilationUnit createAST() {
			return JavaPlugin.getDefault().getASTProvider().getAST(fClassFile, true, null);
		}
		protected IJavaElement getInput() {
			return fClassFile;
		}
		protected IResource getMarkerOwner() {
			return fClassFile.getJavaProject().getProject();
		}
		protected void addSpecialAttributes(Map attributes) throws JavaModelException {
			attributes.put(IDE.EDITOR_ID_ATTR, JavaUI.ID_CF_EDITOR);
			JavaCore.addJavaElementMarkerAttributes(attributes, fClassFile.getType());
			attributes.put(IJavaSearchUIConstants.ATT_JE_HANDLE_ID, fClassFile.getType().getHandleIdentifier());
		}
		protected ISourceReference getSourceReference() {
			return fClassFile;
		}
	}
	private static class FindOccurencesCUEngine extends FindOccurrencesEngine {