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/ASTProvider.java
public void partVisible(IWorkbenchPartReference ref) {
			if (isJavaEditor(ref) && !isActiveEditor(ref))
				activeJavaEditorChanged(ref.getPart(true));
		}
		
		/*
		 * @see org.eclipse.ui.IPartListener2#partInputChanged(org.eclipse.ui.IWorkbenchPartReference)
		 */
		public void partInputChanged(IWorkbenchPartReference ref) {
		}
		/*
		 * @see org.eclipse.ui.IWindowListener#windowActivated(org.eclipse.ui.IWorkbenchWindow)
		 */
		public void windowActivated(IWorkbenchWindow window) {
			IWorkbenchPartReference ref= window.getPartService().getActivePartReference();
			if (isJavaEditor(ref) && !isActiveEditor(ref))
				activeJavaEditorChanged(ref.getPart(true));
		}
		/*
		 * @see org.eclipse.ui.IWindowListener#windowDeactivated(org.eclipse.ui.IWorkbenchWindow)
		 */
		public void windowDeactivated(IWorkbenchWindow window) {
		}
		/*
		 * @see org.eclipse.ui.IWindowListener#windowClosed(org.eclipse.ui.IWorkbenchWindow)
		 */
		public void windowClosed(IWorkbenchWindow window) {
			if (fActiveEditor != null && fActiveEditor.getSite() != null && window == fActiveEditor.getSite().getWorkbenchWindow()) 
				activeJavaEditorChanged(null);
			window.getPartService().removePartListener(this);