Adding TreeViewItems to a TreeView in WPF results in an Exception

In GenerateTreeView you add some items to the parent TreeViewItem. Afterwards, in the InstallerWindow constructor you iterate over these items and add them to another TreeView, e.g. SourceTree. This cannot work since the items can only be contained in one TreeView.

Are you possibly intending this?

public InstallerWindow(string sourcePath, string destPath, List<InstallInstruction> instructions) 
{ 
    InitializeComponent(); 

    GenerateTreeView(SourceTree.Items, sourcePath));
    GenerateTreeView(DestTree.Items, destPath));
}