Problem uploading code

[Jtbillick]

When I upload my program to the practice bot, I am getting an error and I'm not sure how to fix it. This is my first year programming so any tips would be appreciated. This is the error in the Riolog:
➔ Launching «'/usr/local/frc/JRE/bin/java' '-jar' '/home/lvuser/FRCUserProgram.jar'»
➔ Launching «'/usr/local/frc/JRE/bin/java' '-jar' '/home/lvuser/FRCUserProgram.jar'»
ERROR Unhandled exception: java.lang.IllegalArgumentException: Subsystem must not be null. at [edu.wpi.first.wpilibj.command.Command.requires(Com mand.java:173), org.usfirst.frc.team4780.robot.commands.Elevate.<i nit>(Elevate.java:28), org.usfirst.frc.team4780.robot.commands.ElevateUp. <init>(ElevateUp.java:15), org.usfirst.frc.team4780.robot.OI.<init>(OI.java:5 9), org.usfirst.frc.team4780.robot.Robot.robotInit(Rob ot.java:38), edu.wpi.first.wpilibj.IterativeRobot.startCompetit ion(IterativeRobot.java:76), edu.wpi.first.wpilibj.RobotBase.main(RobotBase.jav a:234)]ERROR Unhandled exception: java.lang.IllegalArgumentException: Subsystem must not be null. at [edu.wpi.first.wpilibj.command.Command.requires(Com mand.java:173), org.usfirst.frc.team4780.robot.commands.Elevate.<i nit>(Elevate.java:28), org.usfirst.frc.team4780.robot.commands.ElevateUp. <init>(ElevateUp.java:15), org.usfirst.frc.team4780.robot.OI.<init>(OI.java:5 9), org.usfirst.frc.team4780.robot.Robot.robotInit(Rob ot.java:38), edu.wpi.first.wpilibj.IterativeRobot.startCompetit ion(IterativeRobot.java:76), edu.wpi.first.wpilibj.RobotBase.main(RobotBase.jav a:234)]

WARNING: Robots don't quit!
---> The startCompetition() method (or methods called by it) should have handled the exception above.
WARNING: Robots don't quit!
---> The startCompetition() method (or methods called by it) should have handled the exception above.