To reproduce:

1. Add a RadTreeView and a RadGridView 
2. Use the following code snippet.
3. Select a node from the RadtreeView.
4. Activate the editor for the new row in RadGridView.
5. While the grid editor is active, select a new node in the tree.

public Form1()
{
    InitializeComponent();
    this.radGridView1.MasterTemplate.AddNewBoundRowBeforeEdit = true;
}

public class ClassA 
{
    public int Id { get; set; }

    public string Name { get; set; }

    public string Grade { get; set; }
   
    public ClassA()
    {
    }

    public ClassA(int id, string name, string grade)
    {
        this.Id = id;
        this.Name = name;
        this.Grade = grade;
    }
}

public class ClassB
{
    public int NewID { get; set; }

    public string FirstName { get; set; }

    public string LastName { get; set; }

    public ClassB()
    {
    }

    public ClassB(int id, string firstName, string lastName)
    {
        this.NewID = id;
        this.FirstName = firstName;
        this.LastName = lastName;
    }
}

private void radTreeView1_SelectedNodeChanged(object sender, Telerik.WinControls.UI.RadTreeViewEventArgs e)
{
    if (radGridView1.DataSource is BindingList<ClassA>)
    { 
        this.radGridView1.Columns.Clear();
        this.radGridView1.DataSource = null;
        BindingList<ClassB> list = new BindingList<ClassB>() { new ClassB(1, "John", "Wick") };
        this.radGridView1.DataSource = list;
    }
    else
    {
        this.radGridView1.DataSource = null;
        BindingList<ClassA> list = new BindingList<ClassA>() { new ClassA(1,"John", "A+") };
        this.radGridView1.Columns.Clear();
        this.radGridView1.DataSource = list;
    }
}

Workaround:  this.radGridView1.MasterTemplate.AddNewBoundRowBeforeEdit = false;