handle with undetermined error

    # report_fit(result)

    for param in result.params.values():

        if param.stderr is None: 

            string = "%s:  %f +/- undetermined (init = %f)" % (param.name, param.value, param.init_value)

        else:

            string = "%s:  %f +/- %f (init = %f)" % (param.name, param.value, param.stderr, param.init_value)

        print(string)

    # plot the result

    fig = plt.figure(figsize=(6, 6))

    ax = plt.subplot(1, 1, 1)

    ax.plot(QD_exp,IQ_exp,'.b',label='SANS data')

    ax.plot(QD_exp,IQ_th(result.params, QD),'-r',label='SANS data')

    ax.set_yscale('log')

    ax.set_xscale('log')

    # ax.set_xlim([2,40])

    # ax.set_ylim([0.2,4000])

    ax.set_xlabel(r'$QD$',fontsize=20)

    ax.set_ylabel(r'$I(QD)$',fontsize=20)

    ax.tick_params(direction='in', axis='both', which='both', labelsize=18, pad=10)

    ax.legend(fontsize=16,frameon=False)

    plt.savefig('output.png')

    # plt.show()

    # print the result

    print('Printing results to output.txt')

    with open('/src/output.txt', 'w') as f:

        for param in result.params.values():

            if param.stderr is None: 

                string = "%s:  %f +/- undetermined (init = %f)" % (param.name, param.value, param.init_value)

            else:

                string = "%s:  %f +/- %f (init = %f)" % (param.name, param.value, param.stderr, param.init_value)

            print(string)

            f.write(string)

if __name__ == '__main__':