Create custom ROS msg - Keep getting 'Warning: Unable to save path to file ~/../pathdef.m'

I keep getting the error

 if true
    Warning: Unable to save path to file '/usr/local/MATLAB/R2018a/toolbox/local/pathdef.m'. You can save your path to a
different location by calling SAVEPATH with an input argument that specifies the full path. For MATLAB to use that path
in future sessions, save the path to 'pathdef.m' in your MATLAB startup folder. 
> In savepath (line 176)

I have two custom msgs in my ROS package which I want to use in Matlab now (R2018a, academic use).

My whole code looks like this:

if true
  >> folderpath = '/home/simulator/catkin_ws/src';
  >> rosgenmsg(folderpath)

    Checking subfolder "error_mpc" for custom messages.

    Building custom message files for the following packages:
    error_mpc

    Generating MATLAB classes for message packages in /home/simulator/catkin_ws/src/matlab_gen/jar.

    Loading file error_mpc-0.1.0.jar.
    Generating MATLAB code for error_mpc/Display message type.
    Generating MATLAB code for error_mpc/Error message type.

    To use the custom messages, follow these steps:

    1. Edit javaclasspath.txt, add the following file locations as new lines, and save the file:

    /home/simulator/catkin_ws/src/matlab_gen/jar/error_mpc-0.1.0.jar

    2. Add the custom message folder to the MATLAB path by executing:

    addpath('/home/simulator/catkin_ws/src/matlab_gen/msggen')
    savepath

    3. Restart MATLAB and verify that you can use the custom messages. 
    Type "rosmsg list" and ensure that the output contains the generated
    custom message types.

    >> addpath('/home/simulator/catkin_ws/src/matlab_gen/msggen')
    >> savepath
    Warning: Unable to save path to file '/usr/local/MATLAB/R2018a/toolbox/local/pathdef.m'. You can save           your path to a
different location by calling SAVEPATH with an input argument that specifies the full path. For MATLAB to use that path
in future sessions, save the path to 'pathdef.m' in your MATLAB startup folder. 
> In savepath (line 176) 
  end

How can I write the link to pathdef.m? Or is there a workaround?

Thanks a lot for your help