Problem solved. I was returning the wrong type in a method within the context of my code's logic.

First of all thank you all for downvoting me, it really improves the chances of me leaving an answer. In my case it was a Flutter custom plugin issue, the problem was solved by changing: func speak(_ text: String) { textToSpeech.speak(text) } by func speak(_ text: String, result: @escaping FlutterResult) { textToSpeech.speak(text, result: result) }