the channel (which may be in the middle of being disposed, if error is non-%NULL, error->domain is TP_DBUS_ERRORS and error->code is TP_DBUS_ERROR_PROXY_UNREFERENCED)
%NULL if the channel is ready for use, or the error with which it was invalidated if it is now invalid
Signature of a callback passed to tp_channel_call_when_ready(), which will be called exactly once, when the channel becomes ready or invalid (whichever happens first)