To reproduce: please refer to the attached sample project and try to mouse wheel when no items exist in the drop down.

Workaround:         
public class CustomDropDownList : RadDropDownList
        {
            protected override RadDropDownListElement CreateDropDownListElement()
            {
                return new CustomRadDropDownListElement();
            }

            public override string ThemeClassName  
            { 
                get 
                { 
                    return typeof(RadDropDownList).FullName;  
                }
            }
        }

        public class CustomRadDropDownListElement : RadDropDownListElement
        {
            protected override void HandleSelectNextOrPrev(bool next, bool startFromBeginningIfEndReached)
            {
                if (this.Items.Count == 0)
                {
                    return;
                }
                base.HandleSelectNextOrPrev(next, startFromBeginningIfEndReached);
            }

            protected override Type ThemeEffectiveType     
            { 
                get    
                { 
                    return typeof(RadDropDownListElement);     
                }
            }
        }