"Subscripting into an mxArray is not supported" error in Simulink User Defined Function due to 'fieldnames' extrinsic call

Although I look around the same question on web and see the reasons, I can not find a way to get rid of this error.

I got the error 'Subscripting into an mxArray is not supported' while using Simulink-User Defined Function. My 'extrinsic' call is fieldnames. How can I assign fieldNames(capital 'N', fieldNames=fieldnames(data)) prior to that call? How can I tell the types of fieldNames are strings without subscripting into an mxArray? Because I cannot use fieldNames(i)=...

Note: Or any other suggestion without cell array, fieldname etc? More specifically, I have a .mat file, variable names and their values in it. I will use not only the values, but also their names (strings or char) in my calculations. That is, somehow I should access variable names and values in my Simulink-User Defined Function

function damping = fcn(u)
%#codegen
coder.extrinsic('fieldnames');
coder.extrinsic('length');
coder.extrinsic('cell');
coder.extrinsic('cellstr');
coder.extrinsic('string');

numberOfHydroData = 0;  
i = 0; 
stateProducts = zeros(10,1);
fieldName = ' ';
for k=1:1:13
    for kk=1:1:5
%         fieldNames(k,kk)=' ';  %if uncomment, will give error
    end
end
%%
hydroData = load('data.mat');
hydroData = hydroData.hydroData; % see image below to see how hydroData looks like after implementing this line

numberOfHydroData = length(fieldnames(hydroData))
%% calculation
hydroForces = zeros(6,1);
damping = zeros(6,1);
stateProducts = zeros(int8(numberOfHydroData),1);
fieldNames = cellstr(fieldnames(hydroData));
for i=1:1:numberOfHydroData
      fieldNames(i)                       % just for test below code 
%     fieldName  = char(fieldNames((i))); % if uncomment, will give error
end
% damping = [0;0;0;0;0;0];
damping = hydroForces;
%%

end