Calling a function from another script?

Question

C++ / C#

When creating the Start and Restart buttons, which will appear either at the beginning of the game, and when losing, respectively. Essence of the question - When creating a script, and calling a function from another script in the script, and when calling OnClick on the button, an error occurs:
NullReferenceException: Object reference not set to an instance of an object
PokeyBallTest.Controller.ButtonController.RestartCanvas () (at Assets/Scripts/Controller/ButtonController.cs:20)
UnityEngine.Events.InvokableCall.Invoke () (at :0)
UnityEngine.Events.UnityEvent.Invoke () (at :0) and there's a lot more text...
I will attach a script, tell me how to implement my plan. Thank you!

public class ButtonController : MonoBehaviour
    {
        public GameObject ButtonLose;
        private PlayerController btnLose;

        private void Start()
        {
            btnLose = ButtonLose.GetComponent<PlayerController>();
        }
        public void RestartCanvas()
        {
            btnLose.RestartPlayer();
        }
    }