NullReferenceException

I’m trying to update the text on the screen with this:

    public void Say(string speech, string speaker = "", bool addative = false)
    {
        StopSpeaking();

        SpeechText.text = targetSpeech;

        StartCoroutine(Speaking(speech, addative, speaker));
    }

but I keep getting:
NullReferenceException: Object reference not set to an instance of an object

There’s a post on the top of the forum with how to solve null-refs: https://forum.unity.com/threads/how-to-fix-a-nullreferenceexception-error.1230297/

Thanks