message: added exception for robot nullptr
diff: #include <cinttypes>
  #include <mutex>
  #include <string>
  +#include <stdexcept>
  #include <franka/robot.h>
  #include <joint_limits_interface/joint_limits.h>
  @@ -209,6 +210,10 @@ FrankaHW::FrankaHW(const std::vector<std::string>& joint_names,
  bool franka_hw::FrankaHW::update(
  std::function<bool(const franka::RobotState&)> callback) {
  try {
  + if (!robot_){
  + throw std::invalid_argument("franka::Robot was not "
  + "initialized. Got nullptr instead");
  + } else {
  robot_->read([this, callback](const franka::RobotState& robot_state) {
  robot_state_ = robot_state;
  if (publish_rate_.triggers()) {
  @@ -219,11 +224,13 @@ bool franka_hw::FrankaHW::update(
  }
  return callback(robot_state);
  });
  + }
  } catch (const franka::Exception& e) {
  ROS_ERROR_STREAM("" << e.what());
  return false;
  }
  return true;
  +
  }
  void FrankaHW::publishFrankaStates() {
  