Thanks for sharing the code information.
Based on the shared code, we could see that the you have missed to bind fields property to the ComboBox control. The “Undefined” issue will usually occur if the field are not mapped properly based on the bound dataSource. Kindly refer to the following code
<ej:ComboBox ID="CbFontOfElements" runat="server" DataTextField="Base" OnDataBinding="CbFontOfElements_DataBinding" ></ej:ComboBox>
Kindly make the above changes in your end and ensure the issue.
Let us know if you need any further assistance.