Traceback (most recent call last):


 "/Users/yingwang/anaconda3/envs/toga/lib/python3.7/runpy.py", line 193, in _run_module_as_main

"__main__", mod_spec)


 "/Users/yingwang/anaconda3/envs/toga/lib/python3.7/runpy.py", line 85, in _run_code

exec(code, run_globals)


 "/Users/yingwang/python_projects/my_pycon_sprint_forks/toga-tutorial/toga_file_dialog.py", line 5, in <module>

window.show()


 "/Users/yingwang/python_projects/my_pycon_sprint_forks/toga/src/core/toga/window.py", line 166, in show

self._impl.show()


 "/Users/yingwang/python_projects/my_pycon_sprint_forks/toga/src/cocoa/toga_cocoa/window.py", line 210, in show

self.native.makeKeyAndOrderFront(None)
AttributeError: 'NoneType' object has no attribute 'style'

Traceback (most recent call last):


 "/usr/lib/python3.6/asyncio/events.py", line 126, in _run

self._callback(*self._args)


 "/usr/lib/python3.6/site-packages/txaio/aio.py", line 392, in done

errback(create_failure())


 "/usr/lib/python3.6/site-packages/autobahn/wamp/protocol.py", line 624, in _error

return self._swallow_error(e, errmsg)


 "/usr/lib/python3.6/site-packages/autobahn/wamp/protocol.py", line 432, in _swallow_error

self.log.error(
AttributeError: 'NoneType' object has no attribute 'error'
