Error when saving opened model

[alexeybaran]

I can't save the model, opened from file.

import modelx as mx
m, s = mx.new_model(), mx.new_space()
@mx.defcells
def a():
    return 1
m.save('m.mx')
m2 = mx.open_model('m.mx')
m2.save('m2.mx')

[fumitoh]

Apply this fix https://github.com/fumitoh/modelx/commit/8a5027f893efd3dc7ea60448bb01a92a38fcc2c5