ROS action client sendGoalAndWait error: Java exception occurred: java.lang.​​NullPoint​e​rExcepti​on

My ROS and matlab are both in Ubuntu system. I'm learning sending action goal messages to ROS by matlab. I run example: "Send and Cancel ROS Action Goals" in matlab, but error occurred. It showed "Java exception occurred: java.lang.NullPointerException". I don't know how to solve this problem. So thanks for who can help me.

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[MathWorks Robotics and Autonomous Systems Team]

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Answer 1

[Ivan Belyaev]

Dear Matlab, I have exact the same problem as Ben Lu except that I use FollowJointTrajectory. I have suspicion that some Java or Matlab ROS update destroyed actionclient in matlab. I tried the simple example from https://de.mathworks.com/help/robotics/ref/sendgoal.html. That works fluently, but with the code below it does not work.

Im working on mac 10.12.6 and have R2017b. Same problem had workmate with windows 10 and R2018.

On sendGoal(youbotClient,youbotGoalMsgs) it throws this:

Here is listing of my matlab program

function kuka_simulink_ros_action
  global youbotClient youbotGoalMsgs;

    [youbotClient,youbotGoalMsgs] = rosactionclient('/moveit/arm_controller/follow_joint_trajectory', 'control_msgs/FollowJointTrajectory');
    waitForServer(youbotClient);
    youbotGoalMsgs.Trajectory.JointNames = {'arm_joint_1','arm_joint_2','arm_joint_3','arm_joint_4','arm_joint_5','virtual_theta', 'virtual_x','virtual_y'};
    youbotGoalMsgs.GoalTimeTolerance = rosduration(0,500000000);
    points = rosmessage('trajectory_msgs/JointTrajectoryPoint');
    trajectory = evalin('base', 'youbotTrajectory');
    trj = trajectory.Data;

    for i=1:length(trajectory.Data)
        points(i) = rosmessage('trajectory_msgs/JointTrajectoryPoint');
        points(i).Positions = [trj(i,4),trj(i,5),trj(i,6),trj(i,7),trj(i,8),trj(i,1),trj(i,2),trj(i,3)];
        points(i).Velocities = [0,0,0,0,0,0,0,0];
        points(i).Accelerations = [0,0,0,0,0,0,0,0];
        points(i).TimeFromStart = rosduration(trajectory.Time(i));
    end

    youbotGoalMsgs.Trajectory.Points = points;
    sendGoal(youbotClient,youbotGoalMsgs);
    %delete(youbotClient);
end

PS One month ago exact the same code did worked!

[Ben Lu]

Hello @Ivan Belyaev, I have solved my issue. This is my solution writed in a Chinese blog Matlab generate ROS action message type from ROS packages . You should generate ROS action message that will be used in matlab by yourself from your ROS packages.

1. Install a plugin Robotics System Toolbox Interface for ROS Custom Messages ；

2. Compile your packages in ROS, you will find .msg file, I find the action message generated by matlab was based on these .msg file and package.xml file；

3. Create your message refer to Create Custom Messages from ROS Package 。