I am making a game, and I get the following error:

Object reference not set to an instance of an object.

This is my code:

public Image slotFirst;
bool isFirstEmpty = true;
public Image slotSecond;
bool isSecondEmpty = true;
public Image slotThird;
bool isThirdEmpty = true;

public Sprite keySprite;

public void GotKey() {
    if (isFirstEmpty) {
        slotFirst.sprite = keySprite;
        isFirstEmpty = false;
    }
    else if (isSecondEmpty) {
        slotSecond.sprite = keySprite;
        isSecondEmpty = false;
    }
    else if (isThirdEmpty) {
        slotThird.sprite = keySprite;
        isThirdEmpty = false;
    }
    else {
        Debug.Log("Inventory is full!");
    }
}

Then I put objects in unity like this:

and then I get the error above.

What should I do to solve errors?