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/JavaEditor.java
import org.eclipse.jdt.internal.ui.text.JavaPairMatcher;
import org.eclipse.jdt.internal.ui.text.JavaWordIterator;
import org.eclipse.jdt.internal.ui.text.PreferencesAdapter;
import org.eclipse.jdt.internal.ui.text.java.hover.JavaExpandHover;
import org.eclipse.jdt.internal.ui.util.JavaUIHelp;
import org.eclipse.jdt.internal.ui.viewsupport.ISelectionListenerWithAST;
import org.eclipse.jdt.internal.ui.viewsupport.IViewPartInputProvider;
import org.eclipse.jdt.internal.ui.viewsupport.SelectionListenerWithASTManager;
/**
 * Java specific text editor.
 */
public abstract class JavaEditor extends AbstractDecoratedTextEditor implements IViewPartInputProvider {
	
	/**
	 * Internal implementation class for a change listener.
	 * @since 3.0
	 */
	protected abstract class AbstractSelectionChangedListener implements ISelectionChangedListener  {
		/**
		 * Installs this selection changed listener with the given selection provider. If
		 * the selection provider is a post selection provider, post selection changed
		 * events are the preferred choice, otherwise normal selection changed events
		 * are requested.
		 * 
		 * @param selectionProvider
		 */
		public void install(ISelectionProvider selectionProvider) {
			if (selectionProvider == null)
				return;