Override the ResourceAssemblyInfo property 
The ContentViewDesignerBase class uses a predefined template for its UI. It is located in the Telerik.Sitefinity.Resources assembly. The ContentViewDesignerBase uses the ResourceAssemblyInfo property to locate its resources. You must override this property to return a type form the Telerik.Sitefinity.Resources assembly. 
To do this, perform the following:
    - Add a reference to the Telerik.Sitefinity.Resources assembly.
    You can find the assembly in folder C:\Program Files (x86)\Telerik\Sitefinity CMS\Libraries. 
    - Override the ResourceAssemblyInfo property to return the type of the Reference class.
 
 EXAMPLE: For more information about overriding the ResourceAssemblyInfo class, see For developers: Full code of the multi-view widget designer class.