e estado buscando solución, no encuentro cual es el problema?
File "/home/josue/PycharmProjects/toga/main.py", line 10, in main
app = toga.interface.App('as', 'com.josue.toga', startup=build)
File "/home/josue/entornos_trabajo/toga-taller/lib/python3.6/site-packages/toga/interface/app.py", line 57, in __init__
raise NotImplementedError('App class must define _MAIN_WINDOW_CLASS')
NotImplementedError: App class must define _MAIN_WINDOW_CLASS
File "/home/josue/PycharmProjects/toga/main.py", line 10, in main
app = toga.interface.App('as', 'com.josue.toga', startup=build)
File "/home/josue/entornos_trabajo/toga-taller/lib/python3.6/site-packages/toga/interface/app.py", line 57, in __init__
raise NotImplementedError('App class must define _MAIN_WINDOW_CLASS')
NotImplementedError: App class must define _MAIN_WINDOW_CLASS