Fix null pointer exception in RB Tank drive article (#1739)

Can't get Robot Container joysticks during command static initialization
or constructor since commands are instantiated before Robot Container
constructor finishes. Move getting joysticks to command execution

Author: sciencewhiz

source/docs/software/wpilib-tools/robotbuilder/writing-code/robotbuilder-drive-tank.rst

@@ -334,7 +334,7 @@ Add the Code to do the Driving
       .. code-block:: java
          :linenos:
          :lineno-start: 11
-         :emphasize-lines: 22-23, 51, 57
+         :emphasize-lines: 48, 54
 
          // ROBOTBUILDER TYPE: Command.
 
@@ -357,9 +357,6 @@ Add the Code to do the Driving
 
              // END AUTOGENERATED CODE, SOURCE=ROBOTBUILDER ID=VARIABLE_DECLARATIONS
 
-             private Joystick leftJoystick = RobotContainer.getInstance().getJoystick1();
-             private Joystick rightJoystick = RobotContainer.getInstance().getJoystick2();
-
              // BEGIN AUTOGENERATED CODE, SOURCE=ROBOTBUILDER ID=CONSTRUCTORS
 
 
@@ -386,7 +383,7 @@ Add the Code to do the Driving
              // Called every time the scheduler runs while the command is scheduled.
              @Override
              public void execute() {
-                 m_drivetrain.drive(leftJoystick.getY(), rightJoystick.getY());
+                 m_drivetrain.drive(RobotContainer.getInstance().getJoystick1().getY(), RobotContainer.getInstance().getJoystick2().getY());
              }
 
              // Called once the command ends or is interrupted.