gamultiobj for optimization of two objectives in Simulink

Hello;

I need to use gamultiobj to optimize two objective functions in Simulink. I am using this code for Fitness function:

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function F = Fitness(P)

FlowRate=P; set_param('SolarTroughSystem_Storage_Heating_PV_Opt/constant','Value',num2str(FlowRate)) % Set sim options and compute function value

myobj = sim('SolarTroughSystem_Storage_Heating_PV_Opt','SrcWorkspace','base','StopTime','2289800');

F = (-1) * myobj.get('yout'); end

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The P is the only decision variable that I have. I want to maximize both my objective functions and find a pareto front. If I run this for 200 seconds F comes out as a (201 by 2) matrix. I get the following error:

Error running optimization. Subscripted assignment dimension mismatch.

Could you please help me by this problem?

Thank you so much;

Faezeh