After searching through all the questions related to this, I couldn't find the answer. Most involved people changing the Items property on the TreeViewItem
, which will throw this error.
In my case, I didn't change anything.
However, if you notice.
<TreeViewItem Header="{Binding Header}" ItemsSource="{Binding DataGroups}">
<ListView ItemsSource="{Binding DataGroups}">
I have bound to the same collection twice.
So, double check you haven't bound to the same collection twice if you get this error.