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 ICompilationUnit fCUnit;
		
		public FindOccurencesCUEngine(ICompilationUnit unit, IOccurrencesFinder finder) {
			super(finder);
			fCUnit= unit;
		}
		protected CompilationUnit createAST() {
			return JavaPlugin.getDefault().getASTProvider().getAST(fCUnit, true, null);
		}
		protected IJavaElement getInput() {
			return fCUnit;
		}
		protected IResource getMarkerOwner() throws JavaModelException {
			ICompilationUnit original= WorkingCopyUtil.getOriginal(fCUnit);
			return original.getUnderlyingResource();
		}
		protected void addSpecialAttributes(Map attributes) {
			
		}
		protected ISourceReference getSourceReference() {
			return fCUnit;
		}