Populating ScrollView problem!

Hi guys!

i have a problem to add anything into the scrollview. if you cant find anything wrong in this code feel free to give me a lead! I get the following error = target of get_children is NULL (NULLRefrenceException)

.xaml file

<ContentPage >
   <ScrollView>
      <StackLayout x:Name="PostList">


      </StackLayout>
   </ScrollView>


</ContentPage>

.xaml.cs file

PostList.Children.Add(new Xamarin.Forms.BoxView { BackgroundColor = Color.Black });