wait so u mean there s more to it

[tawny wyvern]

I'm not sure. I'm just saying without analyzing your code for specifics this is my best guess as to why its causing this error.

Yeah what does SecondPhase?.Invoke() do?

[deft sinew]

    {
        FirstBoss.SecondPhaseStarts += Done;
        FirstBoss.SecondPhaseStarts += Again;
    }

    private void OnDisable()
    {
        FirstBoss.SecondPhaseStarts -= Done;
        FirstBoss.SecondPhaseStarts -= Again;
  
    void Done()
    {
        gameObject.SetActive(false);
    }

    void Again()
    {
        gameObject.SetActive(true);
    }

[tawny wyvern]

ah. You may want to use more affirmative assignments

Because these relative assignments could be throwing your values out of whack