The following code exhibits the problem. All you have to do is click the Wrapper column cell and bring down the dropdown. Then select the Phase column. You will get the app to crash in the FilterWrappers method.

namespace RadControlsWinFormsApp2
{
    public partial class Form1 : Form
    {
        private String[] dataPhases = null;
        private String[] dataWrappers = null;
  
        public class Data
        {
            public Data(int ID, string Phase, int Wrapper)
            {
                this.ID = ID;
                this.Phase = Phase;
                this.Wrapper = Wrapper;
            }
            public int ID { get; set; }
            public String Phase { get; set; }
            public int Wrapper { get; set; }
        }
        public Form1()
        {
            InitializeComponent();
  
            this.radGridView1.DataSource = new Data[] { new Data(1, "A", 1), new Data(2, "B", 2) };
        }
  
        private void Form1_Load(object sender, EventArgs e)
        {
            GridViewComboBoxColumn columnPhase = radGridView1.Columns["PHASE"] as GridViewComboBoxColumn;
            dataPhases = new String[] { "A", "B", "C", "A/B/C" };
  
            GridViewComboBoxColumn columnWrapper = radGridView1.Columns["WRAPPER"] as GridViewComboBoxColumn;
            dataWrappers = new String[] { "", "1", "2" };
  
            columnWrapper.DataSource = dataWrappers;
            columnPhase.DataSource = dataPhases;
        }
  
        private void radGridView1_CellEditorInitialized(object sender, Telerik.WinControls.UI.GridViewCellEventArgs e)
        {
            if (e.Column is GridViewComboBoxColumn)
            {
                RadDropDownListEditor editor = e.ActiveEditor as RadDropDownListEditor;
                if (editor != null)
                {
                    RadDropDownListEditorElement editorElement = (RadDropDownListEditorElement)editor.EditorElement;
                    editorElement.Filter = null;
                }
            }
            switch (e.Column.Name)
            {
                case "WRAPPER":
                    {
                        GridViewComboBoxColumn cbc = e.Column as GridViewComboBoxColumn;
                        cbc.DataSource = dataWrappers;
  
                        RadDropDownListEditor editor = e.ActiveEditor as RadDropDownListEditor;
                        if (editor != null)
                        {
                            RadDropDownListEditorElement editorElement = (RadDropDownListEditorElement)editor.EditorElement;
                            editorElement.Filter = FilterWrappers;
                        }
                    }
                    break;
                case "PHASE":
                    {
                        GridViewComboBoxColumn cbc = e.Column as GridViewComboBoxColumn;
                        cbc.DataSource = dataPhases;
                    }
                    break;
            }
        }
  
        private bool FilterWrappers(RadListDataItem item)
        {
            int nWrapper = 0;
            if (item.Value.ToString() != String.Empty)
                nWrapper = Convert.ToInt32(item.Value);
            return true;
        }
    }
}