DNN looks at the supportedfeatures field to get all searchable modules.  Next, for each searchable module, it then looks at the BusinessControllerClass to call it's search function.  If a module's supportedfeature says I'm searchable, then when DNN encounters an empty BusinessControllerClass string, you get your error message.  Setting the BusinessControllerClass to all 0's is not a good idea either.  DNN uses BusinessControllerClass to examine the module's functionality.  If that string is invalid (because there is no BusinessControllerClass that match that name), you will also get these types of error whenever an attempt is made to examine it.