def _check_if_forum_observation_enabled(self):
try:<EOL><INDENT>super()._check_if_forum_observation_enabled()<EOL><DEDENT>except ApiUsageError:<EOL><INDENT>raise ApiUsageError("""<STR_LIT>""")<EOL><DEDENT>
Give a helpful error if the user attempts to subscribe or unsubscribe 
from messages while the token is not registered with a world.  This can 
easily happen if the user attempts to subscribe to messages in the 
constructor.  However, because the constructor is only called on one 
client and message handlers cannot be pickled, subscribing at this time 
would create hard-to-find synchronization bugs.