Leave _frame_objs as empty list (not None) in BaseFigureWidget

Causing error in to_plotly_json

Author: Jon M. Mease

Diff for: plotly/basewidget.py

@@ -131,7 +131,6 @@ def __init__(self,
         # Frames are not supported by figure widget
         if self._frame_objs:
             BaseFigureWidget._display_frames_error()
-        self._frame_objs = None
 
         # Message States
         # --------------