Find the bug in this Command pattern code snippet:

Find the bug in this Command pattern code snippet:

class Receiver {
  void action() { print('Done'); }
}

class Command {
  void execute();
}

class ConcreteCommand implements Command {
  Receiver receiver;
  ConcreteCommand(this.receiver);
  void execute() {
    receiver.action();
  }
}

void main() {
  Command c = ConcreteCommand(null);
  c.execute();
}