incorrect default search_space for GridSearch on OCM

[kmantel]

a = pnl.TransferMechanism(
    name='a',
    function=pnl.Linear(
        slope=(1, pnl.CONTROL),
    )
)

comp = pnl.Composition()
comp.add_node(a)

ocm = pnl.OptimizationControlMechanism(agent_rep=comp)
comp.add_controller(ocm)

produces the error

PsyNeuLink/psyneulink/core/components/functions/nonstateful/optimizationfunctions.py:593: UserWarning: The following arg(s) were not specified for GridSearch Function-0: objective_function, aggregation_function -- using default(s)
  format(self.name, ', '.join(self._unspecified_args)))
Traceback (most recent call last):
  File "", line 1789, in <module>
    agent_rep=comp,
  File "PsyNeuLink/psyneulink/core/globals/context.py", line 737, in wrapper
    return func(*args, context=context, **kwargs)
  File "~/.pyenv/versions/pnl377/lib/python3.7/site-packages/typecheck/decorators.py", line 76, in typecheck_invocation_proxy
    result = method(*args, **kwargs)
  File "PsyNeuLink/psyneulink/core/components/mechanisms/modulatory/control/optimizationcontrolmechanism.py", line 1396, in __init__
    **kwargs
  File "~/.pyenv/versions/pnl377/lib/python3.7/site-packages/typecheck/decorators.py", line 76, in typecheck_invocation_proxy
    result = method(*args, **kwargs)
  File "PsyNeuLink/psyneulink/core/components/mechanisms/modulatory/control/controlmechanism.py", line 1315, in __init__
    **kwargs
  File "PsyNeuLink/psyneulink/core/components/mechanisms/modulatory/modulatorymechanism.py", line 217, in __init__
    **kwargs
  File "PsyNeuLink/psyneulink/core/components/mechanisms/mechanism.py", line 1720, in __init__
    **kwargs
  File "PsyNeuLink/psyneulink/core/components/shellclasses.py", line 90, in __init__
    **kwargs)
  File "PsyNeuLink/psyneulink/core/components/component.py", line 1261, in __init__
    self._instantiate_attributes_after_function(context=context)
  File "PsyNeuLink/psyneulink/core/components/mechanisms/modulatory/control/optimizationcontrolmechanism.py", line 1810, in _instantiate_attributes_after_function
    RANDOMIZATION_DIMENSION: randomization_control_signal_index
  File "PsyNeuLink/psyneulink/core/globals/context.py", line 737, in wrapper
    return func(*args, context=context, **kwargs)
  File "PsyNeuLink/psyneulink/core/components/functions/nonstateful/optimizationfunctions.py", line 1471, in reset
    raise OptimizationFunctionError(f"Invalid {repr(SEARCH_SPACE)} arg for {self.name}{owner_str}; "
psyneulink.core.components.functions.nonstateful.optimizationfunctions.OptimizationFunctionError: Invalid 'search_space' arg for GridSearch Function-0 of OptimizationControlMechanism-0; every dimension must be assigned a SampleIterator.

Automatic setting of a valid search_space is expected