The issue is with the encoder. Because you can use encoders to measure either speed or distance (or both), you need to tell the encoder which measurement to use when the PIDController calls pidGet().
You can do this by adding the line (assuming you want to use distance as your measurement):
Code:
LeftEncoder.setPIDSourceParameter(Encoder.PIDSourceParameter.kDistance);
...after you instantiate the encoder but before you instantiate the PIDController.