'NoneType' object has no attribute 'writeHeaders'

[jiahua]

File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 266, in
inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 122, in callWithContext
return self.currentContext().callWithContext(ctx, func, *args, **kw)
File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 85, in callWithContext
return func(*args,**kw)
File "/usr/lib/python3/dist-packages/twisted/web/wsgi.py", line 533, in run
self.started = True
--- ---
File "/usr/lib/python3/dist-packages/twisted/web/wsgi.py", line 508, in run
self.write(elem)
File "/usr/lib/python3/dist-packages/twisted/web/wsgi.py", line 462, in write
return blockingCallFromThread(
File "/usr/lib/python3/dist-packages/twisted/internet/threads.py", line 122, in blockingCallFromThread
result.raiseException()
File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 467, in raiseException
raise self.value.with_traceback(self.tb)
builtins.AttributeError: 'NoneType' object has no attribute 'writeHeaders'