To reproduces: add a RadTreeView and set up Object-relational hierarchy. Use the following code:

NorthwindEntities context = new NorthwindEntities();

public Form1()
{
    InitializeComponent();

    context.Categories.Load();
    context.Products.Load();

    this.radTreeView1.DataSource = context.Categories.Local.ToBindingList();
    this.radTreeView1.DisplayMember = "CategoryName\\ProductName";
    this.radTreeView1.ChildMember = "Categories\\Products";
}

private void radButton1_Click(object sender, EventArgs e)
{
    this.radTreeView1.ChildMember = "Products\\Categories";
    this.radTreeView1.DisplayMember = "ProductName\\CategoryName"; 
    this.radTreeView1.DataSource = context.Products.Local.ToBindingList();
}

When you click the RadButton, NullReferenceException occurs.

Resolution: Add more descriptive exception message.