Java.lang.NullPointerException

java.lang.NullPointerException

if(arrayTopicos[z][0].equals(arrayDisciplinas[x]) && !existe[z].equals(arrayTopicos[z][1])) {

int x=0;
        int a=0;
        String[] existe;
        for (Object disciplina : arrayDisciplinas) {
            root.add(disci = new DefaultMutableTreeNode(disciplina));
                for (int z=0;z<arrayTopicos.length;z++) {
                    existe = new String[arrayTopicos.length];
                    if(arrayTopicos[z][0].equals(arrayDisciplinas[x]) && !existe[z].equals(arrayTopicos[z][1])) {
                        disci.add(topic = new DefaultMutableTreeNode(arrayTopicos[z][1]));
                        existe[z] = arrayTopicos[z][1];
                    
                        if(arrayItens[a][0].equals(arrayTopicos[a][1])) {
                            topic.add(item = new DefaultMutableTreeNode(arrayItens[a][1]));
                        }
                        a++;
                    }
                }
                x++;
            }