unit – null reference exception when obtaining components

I try to access my enemy and create values ​​of two different classes in a class that tries to change them both. Although, when I do, I continue to have a Null Reference Exception error. The error is where spawnManager is first called.

This is the code that tries to get both values. (And just to note that I'm trying to access these two values ​​from my Welcome scene and that these two values ​​are in my game scene but I do not know if it affects anything):

Private enemy;
SpawnManager private spawnManager;

void Start () {
spawnManager = GetComponent();
enemy = GetComponent();

public void onEasy () {
spawnManager.enemyShipWait = Random.Range (1f, 3f); <- This line has the error
spawnManager.randomPowerupWait = Random.Range (7f, 12f);
enemy.speed = 2.75f;

Here are the two images of my scenes:
Stage of play:

Game scene

Welcome scene:

Welcome scene