The RadExpander element gets underlined in the Visual Studio designer and you can see a NullReferenceException when mouse over the element. The issue occurs only when data bind the IsExpanded property of RadExpander.

There are no issues at runtime. Also, in the common scenario the designer doesn't break. In case the error breaks the designer, you can set the IsExpanded bindining in the code-behind, instead of XAML.

public MainWindow()
{
    InitializeComponent();
    this.radExpander.SetBinding(RadExpander.IsExpandedProperty, new Binding("IsExpanded"));
}