Ensure fitresult.covar is not None before processing

[martinvonk]

I'm running into an issue where the fitresult.covar is None.

Fit succeeded. Could not estimate error-bars.
[[Fit Statistics]]
    # fitting method   = leastsq
    # function evals   = 34
    # data points      = 300
    # variables        = 3
    chi-square         = 22518.4864
    reduced chi-square = 75.8198197
    Akaike info crit   = 1301.49282
    Bayesian info crit = 1312.60417
##  Warning: uncertainties could not be estimated:
    c_0_0:    at initial value
[[Variables]]
    kaq_0_1:  256.584548 (init = 100)
    c_0_0:    1000.00000 (init = 1000)
    Saq_0_1:  7.4078e-05 (init = 0.0001)