Error in add historical event with non-ascii data

[Jon Hill]

Performing a large substitution with the GUI results in:

jhill1@rheic ~/Desktop $ stk-gui SupertreeKD.phyml
/usr/bin/stk-gui:263: PangoWarning: Invalid UTF-8 string passed to pango_layout_set_text()
  gtk.main()
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/stk_gui/interface.py", line 2318, in on_sub_taxa_sub_taxa_button
    XML2 = stk.add_historical_event(XML2,event_desc)
  File "/usr/lib/python2.7/dist-packages/stk/supertree_toolkit.py", line 2238, in add_historical_event
    string.text = event_description
  File "lxml.etree.pyx", line 953, in lxml.etree._Element.text.__set__ (src/lxml/lxml.etree.c:44956)
  File "apihelpers.pxi", line 677, in lxml.etree._setNodeText (src/lxml/lxml.etree.c:20273)
  File "apihelpers.pxi", line 1395, in lxml.etree._utf8 (src/lxml/lxml.etree.c:26485)
ValueError: All strings must be XML compatible: Unicode or ASCII, no NULL bytes or control characters

The CLI works fines.