How to "fix" NullPointerExeption?

Hi , I start Processing and I made a code to play a piano note, i run the code , but when i press the key to play the song , i have an error : NullPointer Exeption and === Minim Error === === Couldn't load the file G.mp3

This is my code : import ddf.minim.*; import ddf.minim.analysis.*; import ddf.minim.effects.*; import ddf.minim.signals.*; import ddf.minim.spi.*; import ddf.minim.ugens.*;

import ddf.minim.*;

Minim minim; // création d'un objet de la classe Minim

AudioPlayer G;

void setup() {

minim = new Minim(this); // 'this' fait référence à l'application en cours

G = minim.loadFile("G.mp3");

}

void draw() {

background(128);

if (key == 'u') G.play(); // key est la dernière touche activée du clavier

if (key == 'p') G.pause();

}

void stop() {

G.close();

minim.stop();

super.stop();

}

I'm french , I hope you to help me asap! :)