Java Compiler reads class as null

Question

I am trying to code an implementation of the A* pathfinding algorithm, and, when searching across an array of States, the compiler outputs the error Exception in thread "main" java.lang.NullPointerException: Cannot invoke "State.toString()" because "nodes[i]" is null at Algorithm.updateGF(Algorithm.java:109) at Algorithm.searchPath(Algorithm.java:27) at Main.main(Main.java:5). This is the subroutine that causes the error

        for (int i = 0; i < nodes.length; i++) {
            if ((openset.containsKey(nodes[i]) && closedset.get(current) < closedset.get(parentset.get(nodes[i])))
                    || (!openset.containsKey(nodes[i]))) {
                parentset.put(nodes[i], current);
                nodes[i].toString();
                openset.put(nodes[i], closedset.get(current) + 1+manhattanDistance(nodes[i],maze.getGoal()));
            }
        }
    }

The same error happens when I try to iterate across a Set<> of States. I have not been able to figure out the cause.

This is the implementation of the State class:

    public enum type{BLANK,OBSTACLE,OBJECTIVE,INITIAL,PATH};
    private int X,Y;
    private type sttype;
    public State(int x, int y, double obsfraction) {
        X = x;
        Y = y;
        double gen = genRandom();
        if(gen<obsfraction) {
            sttype=State.type.OBSTACLE;
        }else {
            sttype=State.type.BLANK;
        }
    }
    private static double genRandom() {
        return Math.random();
    }
    public int getX() {
        return X;
    }
    public int getY() {
        return Y;
    }
    public type getType() {
        return sttype;
    }
    public void setX(int x) {
        X = x;
    }
    public void setY(int y) {
        Y = y;
    }
    public void setType(type sttype) {
        this.sttype = sttype;
    }
    public char toChar() {
        switch (this.getType()) {
        case OBSTACLE:
            return '*';
        case BLANK:
            return ' ';
        case INITIAL:
             return 'I';
        case OBJECTIVE:
            return 'G';
        case PATH:
            return '+';
        default:
            return 'E';
        }
    }
    public String toString() {
        return toChar()+": "+getX()+","+getY();
    }
}

And this is the initialization of openset, parentset, and closedset:

    private static Map<State, Integer> openset = new HashMap<State, Integer>();// Initializes openset as a mapping of
                                                                                // every
    // explored node and its associated g(node)
    private static Map<State, Integer> closedset = new HashMap<State, Integer>();// Initializes closedset as a mapping
                                                                                    // of every
    // explored node and its associated g(node)
    private static Map<State, State> parentset = new HashMap<State, State>();// Initializes parentset as a mapping of
                                                                                // every
    // explored node and its associated parent; that
    // is, the node that leads to it, following the
    // shortest found path
```