运行代码时遇到NullPointerException

Question

我正在模拟一个只有一条跑道的机场。在繁忙时，想要起飞或着陆的飞机必须等待。 使用两个队列实现模拟，每个队列用于等待起飞和降落的飞机。 登陆具有更高的优先级。用户输入命令takeoffflightSymbol，landflightSymbol， 接下来，退出。前两个命令将航班放在适当的队列中。下一个命令 完成当前的起飞或降落，并启动下一个，打印动作（起飞或降落）和 飞行符号。但我似乎无法找到以下代码的问题。救命啊！

public class Runway
{
   public Scanner in;
   public Queue<String> takingOff;
   public Queue<String> landing;

   public Runway()
   {
       in = new Scanner(System.in);
       boolean done = false;
       while (!done)
       {
          System.out.println("Type TAKEOFF/LAND followed by Fight Number to Queue a plane");
          System.out.println("Type NEXT to perform the next action, or QUIT to close the simulation");
          String action = in.next();
          if (action.equals("TAKEOFF"))
          {
              String flight = in.next();
              takingOff.add(flight);
          }
          else if (action.equals("LAND"))
          {
              String flight = in.next();
              landing.add(flight);
          }
          else if (action.equals("NEXT"))
          {
              handleNextAction();
          }
          else if (action.equals("QUIT"))
          {
              System.exit(0);
          }
          else {
              System.out.println("ERROR - not a valid command");
          }
       }
   }

   public void handleNextAction()
   {
       if (landing.size() > 0)
       {
          String flight = landing.remove();
          System.out.println("Flight " + flight + " is landing. ");
       }
       else if (takingOff.size() > 0)
       {
          String flight = takingOff.remove();
          System.out.println("Flight " + flight + " is taking off.");
       }
       else
       {
          System.out.println("There are no flights waiting to take off or land");
       }
   }

   public static void main (String[] args)
   {
       Runway simulator = new Runway();
   }
}

当模拟运行时，除了退出函数之外，所有其他函数都会产生java.lang.NullPointerException错误。