AccessibleJRootPane always returns null for getAccessibleAt()

The subclass of javax.swing.JRootPane (AccessibleJRootPane) that implements the accessibility interface javax.accessibility.AccessibleComponent is derived from java.awt.AccessibleAWTComponent, which returns null for getAccessibleAt() because a component does not necessarily have childs.

But the root pane always has a content pane child, so getAccessibleAt() should be overwritten by AccessibleJRootPane appropriatly.