Why getState() and getTransition doesn't work in unit test?

Question

I have the class below, implemented by me. I want to test with Junit4 the methods. The test of get and set methods are okay (I didn't put them here) but when I try to test the methods getState and getTransition the output is :

java.lang.NullPointerException: Cannot invoke "String.equals(Object)" because the return value of "se21_p2.Nba.nba.State.getStateName()" is null

public class NBA {
    private final ArrayList<State> acceptStateList;
    private final ArrayList<State> stateList;
    private final ArrayList<Transition> transitionList;
    private State initState;

    /**
     * Constructor of NBA
    */
    public NBA(ArrayList<State> acceptStateList, ArrayList<State> stateList, ArrayList<Transition> transitionList, State initState) {
        this.stateList = stateList;
        if (!stateList.contains(initState)){
            addState(initState);
        }
        this.acceptStateList = acceptStateList;
        for (State state: acceptStateList){
            if(!stateList.contains(state))
                addState(state);
        }

        for (State state0: stateList) {
            if (state0.getStateName() != null) {
                for (State state1 : stateList) {
                    if (state1.getStateName() != null) {
                        String s0 = state0.getStateName();
                        String s1 = state1.getStateName();
                        if (stateList.contains(state0) && stateList.contains(state1)) {
                            if (s0.equals(s1))
                                removeState(state1);
                        }
                    }
                }
            }
        }
        this.transitionList = transitionList;
        for (Transition t:transitionList) {
            if(!stateList.contains(t.getStateFrom())){
                addState(t.getStateFrom());
            }
            if (!stateList.contains(t.getStateTo())){
                addState(t.getStateTo());
            }
        }
        this.initState= initState;
    }

  /**
   *  Default constructor of NBA
   */
    public NBA (){
        this.stateList=new ArrayList<>();
        this.acceptStateList= new ArrayList<>();
        this.transitionList= new ArrayList<>();
        this.initState= new State();
    }

  
   
    public State getState(String stateName) {
        State state = new State();
        for (int i = 0; i < stateList.size(); i++) {
            if (stateList.get(i).getStateName().equals(stateName)) {
                state=stateList.get(i);
            }
        }
        return state;
    }
  
    
    public Transition getTransition(String id) {
        Transition t = new Transition();
        for (Transition trans : transitionList) {
            if (trans.getId().equals(id)) {
                t = trans;
            }
        }
        return t;
    }
   
}

State is another class that NBA needs to work. In particular this is the class with the method getStateName(): (there are also other methods for initial and accept but they work)

public class State {
    private String stateName;
    private boolean initial;
    private boolean accept;

    /**
     * Constructor State1
     *
     * @param stateName name of state
     * 
     */
    public State(String stateName) {
        this.stateName = stateName;
        this.initial = false;
        this.accept = false;
    }

    /**
     * Constructor State 2
     */
    public State(String stateName,,boolean initial,boolean accept) {
        this.stateName = stateName;
        this.accept = true;
        this.initial = true;

    }
/*
 *Constructor state 3
*/
    public State(){}

   /**
    * Override equals method
    */
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if(o==null) return false;
        //if(getClass() != o.getClass()) return false;
        if (!(o instanceof State)) return false;
        State state = (State) o;
        if (stateName==null){
            if(state.stateName != null)
                return false;
        }else if(!stateName.equals(state.stateName))
            return false;
        return true;
        }
        //return stateName.equals(state.getStateName());



    @Override
    public int hashCode() {
        return Objects.hash(stateName);
    }

    /**
    * @return stateName name of State
    */
    public String getStateName() {
        return this.stateName;
    }
   /**
    * set name of state
    */
    public void setStateName(String stateName) {
        this.stateName = stateName;
    } 

Is there something wrong? How can I initilize "statename" in the way it is not null?