Could not create instance of type

Question

I have created a UserControl Popup window and called that xaml file into another file. Imported the namespace.


When I am trying to build this project an error at this below given code:

<Popup x:Name="POP" IsOpen="False" PlacementTarget="{Binding}" Placement="Center" AllowsTransparency="True">
      <a:PopUpWindow x:Name="pp" />
</Popup>


Error at :

<a:PopUpWindow x:Name="pp" />
Could not create an instance of type 'PopUpWindow'.