<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def handle_unset_statement_group(self, line: str, position: int, tokens: ParseResults) -> ParseResults: """Unset the statement group, or raises an exception if it is not set. :raises: MissingAnnotationKeyWarning """

        if self.statement_group is None:
            raise MissingAnnotationKeyWarning(self.get_line_number(), line, position, BEL_KEYWORD_STATEMENT_GROUP)
        self.statement_group = None
        return tokens