ccf.marker_colors not available if not set in __init__

[bendichter]

If you construct without marker_colors,

ccf = CCFWidget()
ccf.markers=[np.random.randn(20,3)*1000 for _ in range(5)]

you get markers with the colors automatically assigned:

however, if you try to access these colors:

ccf.marker_colors

you get an error

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
~/opt/anaconda3/lib/python3.7/site-packages/traitlets/traitlets.py in get(self, obj, cls)
    527         try:
--> 528             value = obj._trait_values[self.name]
    529         except KeyError:

KeyError: 'marker_colors'

During handling of the above exception, another exception occurred:

TraitError                                Traceback (most recent call last)
<ipython-input-18-3627e74d660e> in <module>
      1 ccf = CCFWidget()
      2 ccf.markers = [np.random.randn(20, 3) * 1000 for _ in range(5)]
----> 3 ccf.marker_colors
      4 
      5 #= [(1, 0, 0)] * 5

~/opt/anaconda3/lib/python3.7/site-packages/traitlets/traitlets.py in __get__(self, obj, cls)
    554             return self
    555         else:
--> 556             return self.get(obj, cls)
    557 
    558     def set(self, obj, value):

~/opt/anaconda3/lib/python3.7/site-packages/traitlets/traitlets.py in get(self, obj, cls)
    533                 raise TraitError("No default value found for %s trait of %r"
    534                                  % (self.name, obj))
--> 535             value = self._validate(obj, dynamic_default())
    536             obj._trait_values[self.name] = value
    537             return value

~/opt/anaconda3/lib/python3.7/site-packages/traitlets/traitlets.py in _validate(self, obj, value)
    591             value = self.validate(obj, value)
    592         if obj._cross_validation_lock is False:
--> 593             value = self._cross_validate(obj, value)
    594         return value
    595 

~/opt/anaconda3/lib/python3.7/site-packages/traitlets/traitlets.py in _cross_validate(self, obj, value)
    597         if self.name in obj._trait_validators:
    598             proposal = Bunch({'trait': self, 'value': value, 'owner': obj})
--> 599             value = obj._trait_validators[self.name](obj, proposal)
    600         elif hasattr(obj, '_%s_validate' % self.name):
    601             meth_name = '_%s_validate' % self.name

~/opt/anaconda3/lib/python3.7/site-packages/traitlets/traitlets.py in __call__(self, *args, **kwargs)
    905         """Pass `*args` and `**kwargs` to the handler's function if it exists."""
    906         if hasattr(self, 'func'):
--> 907             return self.func(*args, **kwargs)
    908         else:
    909             return self._init_call(*args, **kwargs)

~/opt/anaconda3/lib/python3.7/site-packages/ccfwidget/widget_ccf.py in _validate_marker_colors(self, proposal)
    287     def _validate_marker_colors(self, proposal):
    288         value = proposal['value']
--> 289         self.itk_viewer.point_set_colors = [(1.0, 0.0, 0.0),]*len(self.swc_point_sets) + value
    290         return value
    291 

~/opt/anaconda3/lib/python3.7/site-packages/traitlets/traitlets.py in __set__(self, obj, value)
    583             raise TraitError('The "%s" trait is read-only.' % self.name)
    584         else:
--> 585             self.set(obj, value)
    586 
    587     def _validate(self, obj, value):

~/opt/anaconda3/lib/python3.7/site-packages/traittypes/traittypes.py in set(self, obj, value)
    106 
    107     def set(self, obj, value):
--> 108         new_value = self._validate(obj, value)
    109         old_value = obj._trait_values.get(self.name, self.default_value)
    110         obj._trait_values[self.name] = new_value

~/opt/anaconda3/lib/python3.7/site-packages/traitlets/traitlets.py in _validate(self, obj, value)
    589             return value
    590         if hasattr(self, 'validate'):
--> 591             value = self.validate(obj, value)
    592         if obj._cross_validation_lock is False:
    593             value = self._cross_validate(obj, value)

~/opt/anaconda3/lib/python3.7/site-packages/ipydatawidgets/ndarray/traits.py in validate(self, obj, value)
     23 
     24     def validate(self, obj, value):
---> 25         value = super(NDArray, self).validate(obj, value)
     26         if value is None or value is Undefined:
     27             return value

~/opt/anaconda3/lib/python3.7/site-packages/traittypes/traittypes.py in validate(self, obj, value)
    103         except (ValueError, TypeError) as e:
    104             raise TraitError(e)
--> 105         return super(Array, self).validate(obj, value)
    106 
    107     def set(self, obj, value):

~/opt/anaconda3/lib/python3.7/site-packages/traittypes/traittypes.py in validate(self, obj, value)
     74         try:
     75             for validator in self.validators:
---> 76                 value = validator(self, value)
     77             return value
     78         except (ValueError, TypeError) as e:

~/opt/anaconda3/lib/python3.7/site-packages/ipydatawidgets/ndarray/traits.py in validator(trait, value)
     39         if len(value.shape) != len(args):
     40             raise TraitError('%s shape expected to have %s components, but got %s components' % (
---> 41                 trait.name, len(args), value.shape))
     42         for i, constraint in enumerate(args):
     43             if constraint is not None:

TraitError: point_set_colors shape expected to have 2 components, but got (0,) components

Setting ccf.marker_colors works as expected, this is just an issue with getting the attr