NullPointerException at drivecode

[r0306]

When we run our drive code, it throws the following error:
[cRIO] java.lang.NullPointerException
[cRIO] at edu.wpi.first.wpilibj.RobotDrive.arcadeDrive(Robot Drive.java:342)
[cRIO] at edu.wpi.first.wpilibj.RobotDrive.arcadeDrive(Robot Drive.java:354)
[cRIO] at Robot2013.RobotMain.teleopPeriodic(RobotMain.java: 90)
[cRIO] at edu.wpi.first.wpilibj.IterativeRobot.startCompetit ion(IterativeRobot.java:165)
[cRIO] at edu.wpi.first.wpilibj.RobotBase.startApp(RobotBase .java:169)
[cRIO] in virtual method #10 of javax.microedition.midlet.MIDlet(bci=17)
[cRIO] at javax.microedition.midlet.MIDletTunnelImpl.callSta rtApp(64)
[cRIO] at com.sun.squawk.imp.MIDletMainWrapper.main(110)
[cRIO] in virtual method #95 of com.sun.squawk.Klass(bci=25)
[cRIO] at com.sun.squawk.Isolate.run(1506)
[cRIO] at java.lang.Thread.run(231)
[cRIO] in virtual method #47 of com.sun.squawk.VMThread(bci=42)
[cRIO] in static method #3 of com.sun.squawk.VM(bci=6)

public class RobotMain extends IterativeRobot
{

public static RobotDrive driveTrain;
public static Joystick leftStick;

public void robotInit()
{

driveTrain = new RobotDrive(4, 3, 1, 2);

driveTrain.setSafetyEnabled(false);

}


public void autonomousPeriodic()
{

}

public void teleopPeriodic()
{

driveTrain.arcadeDrive(leftStick); //line 90

}

public void testPeriodic() {

}
}

The motors aren't running at all and I don't get what could've gone wrong here as the code seems fine.

[MagiChau]

You never instantiated the joystick so there is no Joystick to get any values from.

Joystick leftStick = new Joystick(1);