How can I set placement of popup to "MousePoint"?

[SPE]

Here's my snippet.

 <Popup x:Name="parentPopup"
               telerik:PopupExtensions.PlacementTarget="{Binding ElementName=accountGridView}"
               telerik:PopupExtensions.Placement="MousePoint"
               telerik:PopupExtensions.StaysOpen="False">


I expected that this popup should be opened at the same position of mousepoint.

However, If I do not set the PlacementTarget property, it throws an exception (target null).

And, when I set the position, like above snippet, the position of popup changes time to time.

Moreover, I cannot understand why I should set the PlacementTarget property when I use "Placement=MousePoint".


How can I use this property?

Thank you in advance.