ipython: Tab-completion raises exception

Not sure whether this is an issue with IPython 5.0.0 or pyramid_ipython 0.1, but tab-completion raises an exception when trying to tab-complete the transaction module.

1. pip install -U ipython pyramid_ipython transaction
2. pshell development.ini
3. import tran<TAB>

   In [5]: import tranException in thread Thread-127:
   Traceback (most recent call last):
   File "/usr/lib64/python2.7/threading.py", line 813, in __bootstrap_inner
       self.run()
   File "/usr/lib64/python2.7/threading.py", line 766, in run
       self.__target(_self.__args, *_self.__kwargs)
   File "/usr/lib/python2.7/site-packages/prompt_toolkit/interface.py", line 835, in run
       completions = list(buffer.completer.get_completions(document, complete_event))
   File "/usr/lib/python2.7/site-packages/IPython/terminal/ptutils.py", line 24, in get_completions
       cursor_pos=document.cursor_position_col
   File "/usr/lib/python2.7/site-packages/IPython/core/completer.py", line 84, in comp
       text, matches =  complete(_args, *_kwargs)
   File "/usr/lib/python2.7/site-packages/IPython/core/completer.py", line 1186, in complete
       custom_res = self.dispatch_custom_completer(text)
   File "/usr/lib/python2.7/site-packages/IPython/core/completer.py", line 1110, in dispatch_custom_completer
       res = c(event)
   File "/usr/lib/python2.7/site-packages/IPython/core/completerlib.py", line 253, in module_completer
       return module_completion(event.line)
   File "/usr/lib/python2.7/site-packages/IPython/core/completerlib.py", line 229, in module_completion
       return get_root_modules()
   File "/usr/lib/python2.7/site-packages/IPython/core/completerlib.py", line 119, in get_root_modules
       rootmodules_cache = ip.db.get('rootmodules_cache', {})
   AttributeError: 'NoneType' object has no attribute 'db'

@mmerickel