Amazon Polly `builder.continueOnSuccessWith` in iOS not responding

Question

I want to integrate the AWS polly service to my iOS application for text to speech purpose. I have setup the code that is available on GitHub as starter project. I have setup the CognitoIdentityPoolId in AWS console correctly.

This is the code that is use for getting the URL of inputed text but the closure builder.continueOnSuccessWith that is responsible for getting url is not running or giving any error.

let input = AWSPollySynthesizeSpeechURLBuilderRequest()

        // Text to synthesize
        input.text = "Sample text"

        // We expect the output in MP3 format
        input.outputFormat = AWSPollyOutputFormat.mp3

        // Choose the voice ID
        input.voiceId = AWSPollyVoiceId.joanna

        // Create an task to synthesize speech using the given synthesis input
        let builder = AWSPollySynthesizeSpeechURLBuilder.default().getPreSignedURL(input)

        // Request the URL for synthesis result
        builder.continueOnSuccessWith(block: { (awsTask: AWSTask<NSURL>) -> Any? in
            // The result of getPresignedURL task is NSURL.
            // Again, we ignore the errors in the example.
            let url = awsTask.result!

            // Try playing the data using the system AVAudioPlayer
            self.audioPlayer.replaceCurrentItem(with: AVPlayerItem(url: url as URL))
            self.audioPlayer.play()
            debugPrint("Player success")
            return nil
        })

I am not getting any error in console as well. How to debug where is the issue ??