JUnit5 getting NullPointerException when expecting another kind of exception

I’m trying to test a simple class, where I’m testing a case where if I put a string into a method, where the method is not part of a certain list (sidearms), it should return an illegalArguementException. In this example: "Not Desert Eagle" is not a part of the array list sidearms. But why is the test failing and returning that it expected an IllegalArguementException but was a in fact a nullPointerException?

The test class:

package RandomClassGenerator;

import static org.junit.jupiter.api.Assertions.assertThrows;

import org.junit.jupiter.api.Test;

public class SidearmTest {
    
    private Sidearm sideArm;
    private String sidearm = "NOT Desert Eagle";

    @Test
    public void testSidearm() {
        assertThrows(IllegalArgumentException.class, () -> {
            sideArm.sidearmAttachment(sidearm);
        });
        
    }

The class being tested:

import java.util.Arrays;
import java.util.List;

public class Sidearm implements IGiveRandom{
    
    // This class gives the player a sidearm with/without an attachment, as long as Overkill is not a given perk

    private String sidearm;
    private String sidearmAttachment;

    private List<String> sidearms = Arrays.asList("M9","USP.45","M1911.45","Desert Eagle");
    private List<String> sidearmAttachments = Arrays.asList("None","Silencer");

    // Constructor:
    public Sidearm() {
        sidearm = sidearm();
        sidearmAttachment = sidearmAttachment(sidearm);
    }
    // Gives a random sidearm from the sidearms array list
    public String sidearm() {
        int randomFromList = rand.nextInt(sidearms.size());
        sidearm = sidearms.get(randomFromList);
        return sidearm;
    }
    // Randomely, either gives or not, an attachment for the sidearm:
    public String sidearmAttachment(String weapon) {
        // The Desert Eagle has no attachments
        if (!(sidearms.contains(weapon))) throw new IllegalStateException("Sidearm not found");
        if (sidearm.equals("Desert Eagle")) {
            sidearmAttachment = "None";
        } else {
            // The rest of the sidearms can either have a silencer or no attachment
            int randomFromList = rand.nextInt(sidearmAttachments.size());
            sidearmAttachment = sidearmAttachments.get(randomFromList);
        }
        return sidearmAttachment;
    }

>Solution :

why is the test failing and returning that it expected an IllegalArguementException but was a in fact a nullPointerException?

Because variable sideArm still has its default value (null) in the test, when you try to invoke sidearmAttachment() on it. You need to create a Sidearm for it to refer to, that will be the subject of the test.