NullReferenceException is thrown, when the following steps are performed:

1. Scroll horizontally
2. Move a column by mouse drag and drop.
3. Start writing in the filter cell of the moved column
4. Suddenly the editor is closed.
5. When you try to write in the filter cell again, exception is thrown.

Work around:

void radGridView1_EditorRequired(object sender, Telerik.WinControls.UI.EditorRequiredEventArgs e)
{
    if (e.EditorType == typeof(RadTextBoxEditor))
    {
        e.Editor = new RadTextBoxEditor();
    }
}