Question PlayerPrefs NullReferenceException error

It works for the first level, after the level is finished I use it to display the time, but the second and third times does not work. I get "NullReferenceException: Object reference not set to an instance of an object" when I finish the level. Any ideas why?

[SerializeField]
private Cronometru VaribileCronometru; // I use this to acces Cronometrule from Cronometru script

private void OnTriggerEnter(Collider other)
{
if (other.gameObject.CompareTag("Pick Up"))
{
PlayerPrefs.SetString("YourTime", VaribileCronometru.Cronometrule.text); // here it works
SceneManager.LoadScene("finishlvl1");
}
if (other.gameObject.CompareTag("Pick Up2"))
{
PlayerPrefs.SetString("YourTime", VaribileCronometru.Cronometrule.text); // error occurs here
SceneManager.LoadScene("finishlvl2");
}
if (other.gameObject.CompareTag("Pick Up3"))
{
PlayerPrefs.SetString("YourTime", VaribileCronometru.Cronometrule.text); // error occurs here
SceneManager.LoadScene("finishlvl3");
}
}

 

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Thanks! Is my first post here.