Catter for empty docstrings (bug introduced 2 checkins ago).

IPython/frontend/wx/wx_frontend.py

@@ -233,6 +233,8 class WxController(ConsoleWidget, PrefilterFrontEnd):
             for name in symbol_string.split('.')[1:] + ['__doc__']:
                 symbol = getattr(symbol, name)
             self.AutoCompCancel()
+            # Check that the symbol can indeed be converted to a string:
+            symbol += ''
             wx.CallAfter(self.CallTipShow, self.GetCurrentPos(), symbol)
         except:
             # The retrieve symbol couldn't be converted to a string