Visitor for `FunctionBody` AST node.

def visit_FunctionBody(self, node):
        """Visitor for `FunctionBody` AST node."""
        for child in node.children:
            return_value = self.visit(child)

            if isinstance(child, ReturnStatement):
                return return_value

            if isinstance(child, (IfStatement, WhileStatement)):
                if return_value is not None:
                    return return_value

        return NoneType()