NullReferenceException: Object reference not set to an instance of an object _

Question

Code:

    if (StoneScript.scoreTex.text != "Pickaxe")
    {
        if(GrassScript.amount >= 3)
        {
            GrassScript.amount = GrassScript.amount - 3;
            GrassScript.scoreText.text = "Wood: " + GrassScript.amount;
            StoneScript.scoreTex.text = "Pickaxe";
        }
    }

It works earlier but now won't.

In StoneScript.cs:

public static Text scoreText;
public static Text scoreTex;
public static int amount = 0;