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
* Jumps to the matching bracket.
	 */
	public void gotoMatchingBracket() {
		
		ISourceViewer sourceViewer= getSourceViewer();
		IDocument document= sourceViewer.getDocument();
		if (document == null)
			return;
		
		IRegion selection= getSignedSelection(sourceViewer);
		int selectionLength= Math.abs(selection.getLength());
		if (selectionLength > 1) {
			setStatusLineErrorMessage(JavaEditorMessages.getString("GotoMatchingBracket.error.invalidSelection"));			
			sourceViewer.getTextWidget().getDisplay().beep();
			return;
		}
		
		int sourceCaretOffset= selection.getOffset() + selection.getLength();
		if (isSurroundedByBrackets(document, sourceCaretOffset))
			sourceCaretOffset -= selection.getLength();
		IRegion region= fBracketMatcher.match(document, sourceCaretOffset);
		if (region == null) {
			setStatusLineErrorMessage(JavaEditorMessages.getString("GotoMatchingBracket.error.noMatchingBracket"));			
			sourceViewer.getTextWidget().getDisplay().beep();
			return;		
		}
		
		int offset= region.getOffset();
		int length= region.getLength();