-(void)startRecognizer {
    recognitionTask = [speechRecognizer recognitionTaskWithRequest:reco resultHandler:^(SFSpeechRecognitionResult * _Nullable result, NSError * _Nullable error) {
             //result = ok
        }];
        self.recognitionRequest = reco;
}

recognitionTask = nil

-(void)startRecognizer {
    recognitionTask = [speechRecognizer recognitionTaskWithRequest:reco resultHandler:^(SFSpeechRecognitionResult * _Nullable result, NSError * _Nullable error) {
             //result = nil...
        }];
        self.recognitionRequest = reco;
}