an other crash, YEAH

Traceback (most recent call last):
File "poezio.py", line 81, in
main()
File "poezio.py", line 78, in main
gui.main_loop(stdscr)
File "/home/louiz/workspace/poezio/src/gui.py", line 133, in main_loop
self.execute()
File "/home/louiz/workspace/poezio/src/gui.py", line 503, in execute
func(args)
File "/home/louiz/workspace/poezio/src/gui.py", line 555, in command_win
self.window.refresh(self.rooms)
File "/home/louiz/workspace/poezio/src/window.py", line 615, in refresh
self.text_win.refresh(room)
File "/home/louiz/workspace/poezio/src/window.py", line 256, in refresh
lines = self.build_lines_from_messages(messages)
File "/home/louiz/workspace/poezio/src/window.py", line 214, in build_lines_from_messages
if txt[:self.width-offset].find('\n') != -1:
TypeError: 'NoneType' object is unsubscriptable

Reproduce :
receive a NEW private message (window was not opened before this message) with a \n