A Node object that indicates the previous visible node.
Use the PrevVisibleNode property to get the previous
visible node. Use the NextVisibleNode
property to get the next visible node. Use the Visible
property to hide a node. Use the FirstVisibleNode
property to get the first visible node in the control's client area. If there
is no previous tree node, the PrevVisibleNode property returns a null
reference (Nothing in Visual Basic).