The result of the callback.
Exceptions thrown in ICallbackEventHandler.RaiseCallbackEvent(string) will not prevent ICallbackEventHandler.GetCallbackResult from being called and returning a result. To prevent ICallbackEventHandler.GetCallbackResult from returning a result, set a flag when an exception is thrown in ICallbackEventHandler.RaiseCallbackEvent(string) and return an empty string from ICallbackEventHandler.GetCallbackResult.