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
String key= preference == null ? null : preference.getIsGoToNextNavigationTargetKey();
		return (key != null && preferences.getBoolean(key));
	}
	/**
	 * Computes and returns the source reference that includes the caret and
	 * serves as provider for the outline page selection and the editor range
	 * indication.
	 * 
	 * @return the computed source reference
	 * @since 3.0
	 */
	protected ISourceReference computeHighlightRangeSourceReference() {
		ISourceViewer sourceViewer= getSourceViewer();
		if (sourceViewer == null)
			return null;
			
		StyledText styledText= sourceViewer.getTextWidget();
		if (styledText == null)
			return null;
		
		int caret= 0;
		if (sourceViewer instanceof ITextViewerExtension5) {
			ITextViewerExtension5 extension= (ITextViewerExtension5)sourceViewer;
			caret= extension.widgetOffset2ModelOffset(styledText.getCaretOffset());
		} else {
			int offset= sourceViewer.getVisibleRegion().getOffset();
			caret= offset + styledText.getCaretOffset();
		}
		IJavaElement element= getElementAt(caret, false);