Re-importing variables breaks .xreplace

[schymans]

In essm/docs/examples/api_features.ipynb, at the end of the notebook, where variables are re-imported using from test_variable_definitions import *, the following gives an error:

cdict = Variable.__defaults__.copy()
cdict[Pr] = 0.71   # Prandtl number for air
vdict = {P_a:101325., P_wa: 2000., R_s: 900., Re_c:3000., T_a:300., v_w: 0.5}
vdict.update(cdict)

eq_Pa.xreplace(vdict)

The error is SympifyError: SympifyError: None
The same code can be run without problems before the re-import of variables.