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/viewsupport/SelectionListenerWithASTManager.java
firePostSelectionChanged((ITextSelection) selection);
					}
				}
			};
		}
		public boolean isEmpty() {
			return fAstListeners.isEmpty();
		}
		public void install(ISelectionListenerWithAST listener) {
			if (isEmpty()) {
				ISelectionProvider selectionProvider= fPart.getSelectionProvider();
				if (selectionProvider instanceof IPostSelectionProvider) {
					((IPostSelectionProvider) selectionProvider).addPostSelectionChangedListener(fPostSelectionListener);
					selectionProvider.addSelectionChangedListener(fSelectionListener);
				}
			}
			fAstListeners.add(listener);
		}
		
		public void uninstall(ISelectionListenerWithAST listener) {
			fAstListeners.remove(listener);
			if (isEmpty()) {
				ISelectionProvider selectionProvider= fPart.getSelectionProvider();
				if (selectionProvider instanceof IPostSelectionProvider) {
					((IPostSelectionProvider) selectionProvider).removePostSelectionChangedListener(fPostSelectionListener);
					selectionProvider.removeSelectionChangedListener(fSelectionListener);
				}
			}
		}