Pickling Theme errors on v7.0.0

[notmatthancock]

We recently upgraded traitsui: 6.1.3-5 --> 7.0.0-1. The following code fails on the newer version but runs fine on the older:

>>> import pickle
>>> from traitsui.theme import Theme
>>> theme = Theme()
>>> pickle.dumps(theme)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "...traits/has_traits.py", line 1230, in __reduce_ex__
    return (__newobj__, (self.__class__,), self.__getstate__())
  File "...traits/has_traits.py", line 1194, in __getstate__
    result = self.trait_get(transient=is_none)
  File "...traits/has_traits.py", line 1298, in trait_get
    value = getattr(self, name, missing)
  File "...traitsui/theme.py", line 69, in _get_content_color
    import wx
ModuleNotFoundError: No module named 'wx'

Full context

We have a tree editor for which some nodes have color icons. The color nodes specifically fail to be draggable (into an IPython shell for example) because of the pickling failure above.

[corranwebster]

I think the immediate fix for this is easy (fix the methods so they aren't wx-specific), but I'm also fairly comfortable saying that when a tree node is serialized for drag and drop, we shouldn't be accidentally getting a Theme object along for the ride. Can you give a little more context of your set up.

Also, Themes really, really, really need to die, but that's secondary.

[notmatthancock]

Can you give a little more context of your set up.

Hmm. Besides the example above, I'm having a hard time reproducing this outside of our application.

Some more general notes:

1. The occurs when dragging items out of a view with a tree editor, who has multiple custom tree nodes for various model types
2. It only seems to occur for a tree node of a certain type, but I can't seem to associate a difference with the error
3. We are not explicitly using anything from Theme

I need to investigate more along the lines of 2., but do not have the time at the moment.