WPF Listview Header Click Error

I have a gridview in the listview. When I click on the headers in the gridview(Name,Surname,Position) then it gives me the following error. 

My XAML Code is below.

<local:ListViewExt Height="363" x:Name="vorstandlist"  Width="391" Margin="6,32,423,0" DataContext="{Binding}" ItemsSource="{Binding}" MouseDoubleClick="vorstandlist_MouseDoubleClick"  ><ListView.View ><GridView ><GridViewColumn Width="140" Header="Name" DisplayMemberBinding="{Binding Path=name}" /><GridViewColumn Width="140" Header="Surname" DisplayMemberBinding="{Binding Path=Surname}" /><GridViewColumn Width="140" Header="Position" DisplayMemberBinding="{Binding Path=FPosition}" /></GridView></ListView.View>

KAdir