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
* @see org.eclipse.ui.IPartListener2#partClosed(org.eclipse.ui.IWorkbenchPartReference)
		 */
		public void partClosed(IWorkbenchPartReference ref) {
			if (isActiveEditor(ref))
				activeJavaEditorChanged(null);
		}
		
		/*
		 * @see org.eclipse.ui.IPartListener2#partDeactivated(org.eclipse.ui.IWorkbenchPartReference)
		 */
		public void partDeactivated(IWorkbenchPartReference ref) {
		}
		
		/*
		 * @see org.eclipse.ui.IPartListener2#partOpened(org.eclipse.ui.IWorkbenchPartReference)
		 */
		public void partOpened(IWorkbenchPartReference ref) {
			if (isJavaEditor(ref) && !isActiveEditor(ref))
				activeJavaEditorChanged(ref.getPart(true));
		}
		
		/*
		 * @see org.eclipse.ui.IPartListener2#partHidden(org.eclipse.ui.IWorkbenchPartReference)
		 */
		public void partHidden(IWorkbenchPartReference ref) {
		}
		
		/*
		 * @see org.eclipse.ui.IPartListener2#partVisible(org.eclipse.ui.IWorkbenchPartReference)
		 */