When is the UpdateManager definitely available?

Hi I am using the following code:

canvas.addGVTTreeRendererListener(new GVTTreeRendererAdapter() {
        @Override
        public void gvtRenderingCompleted(GVTTreeRendererEvent e) {
          canvas.getUpdateManager().getUpdateRunnableQueue().invokeLater(runnable);
        }
      });

to do some changes in the DOM of an SVG document. I expected that the
updatemanager is definitely available when gvtRenderingCompleted was
called.
However, it now happened at least once that I got a NullPointerException
on the line
  canvas.getUpdateManager().getUpdateRunnableQueue().invokeLater(runnable);

So it seems that there is still no guarantee that the UpdateManager is
available then. Am I missing something? Must I register on a different
listener?

Regards
Marco