Calculations return 'None' value

Hey!

During certain calculations, the results are showing 'None' return. 

`xf.find_coefficients(airfoil = 'naca0012', alpha = 25, Reynolds = 1e6, iteration = 100)`
`{'alpha': None, 'CL': None, 'CD': None, 'CDp': None, 'CM': None, 'Top_Xtr': None, 'Bot_Xtr': None}`

Is this because of solving issues or seperation domain?

Thanks!