Thanks for writing again.
After examining the SetPosition() method it turns out there is only one place where it can crash with a null reference exception.
.pointer.Opacity = 1;
UIElement parent =
.page = ElementTreeHelper.FindVisualAncestor<PhoneApplicationPage>(
// Calculate position relative to the tooltip owner.
Notice how the parent variable cannot be the cause since it is never dereferenced. ElementTreeHelper also can't be the cause since it is a static class. Also it does not crash inside FindVisualAncestor() because the stack trace you posted ends with the SetPosition() method. With that said, the only possible place for a null reference exception to occur is when the page's RenderSize is accessed. This means that ElementTreeHelper returned null because no page exists in the Silverlight element tree at this moment.
Please clarify at which point of your application's life-cycle does the exception occur. You can try opening the tooltip when you are sure your application has created the current page. We will see what we can do on our side in order to prevent this exception.
Thanks for the feedback. Your Telerik points have been updated for this bug report.
the Telerik team