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
* @see org.eclipse.core.runtime.jobs.Job#run(org.eclipse.core.runtime.IProgressMonitor)
				 * @since 3.0
				 */
				protected IStatus run(IProgressMonitor monitor) {
					CompilationUnit ast= JavaPlugin.getDefault().getASTProvider().getAST(inputElement, true, null);
					if (fOverrideIndicatorManager != null) 
						fOverrideIndicatorManager.reconciled(ast, true, monitor);
					return Status.OK_STATUS;
				}
			};
			job.setPriority(Job.DECORATE);
			job.setSystem(true);
			job.schedule();
		}
	}
	
	/**
	 * Tells whether override indicators are shown.
	 * 
	 * @return <code>true</code> if the override indicators are shown
	 * @since 3.0
	 */
	protected boolean isShowingOverrideIndicators() {
		AnnotationPreference preference= getAnnotationPreferenceLookup().getAnnotationPreference(OverrideIndicatorManager.ANNOTATION_TYPE);
		IPreferenceStore store= getPreferenceStore();
		return getBoolean(store, preference.getHighlightPreferenceKey())
			|| getBoolean(store, preference.getVerticalRulerPreferenceKey())
			|| getBoolean(store, preference.getOverviewRulerPreferenceKey())
			|| getBoolean(store, preference.getTextPreferenceKey());
	}