How to solve this proplem in Java. Output Showing below

Exception in thread "main" java.lang.NullPointerException

at MazeGUI.(MazeGUI.java:34)

at MazeGUI.main(MazeGUI.java:40)

The code: public MazeGUI(){ this.setTitle("My GUI Maze"); this.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE); this.setSize(700,700); panGrid.setLayout(new GridLayout(SIZE, SIZE)); initializeGrid(); this.add(panGrid); this.setVisible(true); }

public static void main(String[] args){ new MazeGUI(); }