This error raises when creating an evaluation with an unsaved patient:

Traceback (most recent call last):
  File "/.local/lib/python3.7/site-packages/trytond/model/model.py", line 368, in _getitem
    return getattr(self, field)
AttributeError: age

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/.local/lib/python3.7/site-packages/trytond/model/model.py", line 291, in _getattr_
    return self._values[name]
  File "/.local/lib/python3.7/site-packages/trytond/model/model.py", line 370, in _getitem
    raise KeyError(field)
KeyError: 'age'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/.local/lib/python3.7/site-packages/trytond/wsgi.py", line 117, in dispatch_request
    return endpoint(request, **request.view_args)
  File "/.local/lib/python3.7/site-packages/trytond/protocols/dispatcher.py", line 47, in rpc
    request, database_name, *request.rpc_params)
  File "/.local/lib/python3.7/site-packages/trytond/wsgi.py", line 84, in auth_required
    return wrapped(args, *kwargs)
  File "/.local/lib/python3.7/site-packages/trytond/protocols/wrappers.py", line 159, in wrapper
    return func(request, pool, args, *kwargs)
  File "/.local/lib/python3.7/site-packages/trytond/protocols/dispatcher.py", line 185, in _dispatch
    result = rpc.result(meth(inst, c_args, *c_kwargs))
  File "/.local/lib/python3.7/site-packages/trytond/model/modelview.py", line 88, in wrapper
    result = func(self, args, *kwargs)
  File "/.local/lib/python3.7/site-packages/trytond/model/fields/field.py", line 123, in wrapper
    return func(self, args, *kwargs)
  File "/.local/lib/python3.7/site-packages/trytond/modules/health/health.py", line 5019, in on_change_patient
    self.computed_age = self.patient.age
  File "/.local/lib/python3.7/site-packages/trytond/model/modelstorage.py", line 1485, in _getattr_
    return super(ModelStorage, self).__getattr__(name)
  File "/.local/lib/python3.7/site-packages/trytond/model/model.py", line 294, in _getattr_
    % (self.__name__, name, self._values))
AttributeError: 'gnuhealth.patient' Model has no attribute 'age': <trytond.model.model.gnuhealth.patient._record object at 0x7fb4e6f1d208>

While it is in currently not possible to create evaluations outside of a patient domain in Standard GNU Health (thus always requiring to save the patient beforehand) the model should nevertheless work in different contexts.

A quick fix is to skip the evaluation of unavailable properties with

    @fields.depends('patient')
    def on_change_patient(self):
        if self.patient.id > 0:
            ...