Hi Francisco,
The GSLSimAnMinimizer does not implement the computation of errors , since the minimiser does not use any derivatives of the objective functions.
I should add an error message in the code to avoid this crash.
The way to compute errors in that case is if you can try running one of the other minimizer from the minimum point, or if you cannot use a different procedure like bootstrapping, i.e. generate different datasets similar to the one you have and find the optimal parameters for each one of them. From the variation obtained you can estimate then the parameter errors.
Best regards
Lorenzo