Cannot start coroutine within method called by UnityEvent if Singleton is used (UI Button)

Unity Engine
,
1

Unity 2021.3.24

I am trying to call a method, which starts a coroutine.
When the class I use is a singleton, and I start a coroutine in the called function, I get the error Trying to Invoke method: SongManager.StartSong couldn't be called.

When I remove StartCoroutine or remove the singleton, it does work, but I would like to use the singleton pattern for this class if possible.

Does anyone know why this happens and how to fix it (except removing singleton)?

Code (SingInfo is a scriptable Object, which is referenced in the event):

public void StartSong(SongInfo song)
    {
        songInfo = song;
        timer = 0;

        StartCoroutine(PlaySong());
    }
2

Solved, the tools I use used Invoke