function cross_chain_erc20_settlement(
    string memory to_chain,
    uint256 to_handler,
    uint256 to_token,
    uint256 to,
    uint256 amount
) external {
    require(amount > 0, "Amount must be greater than 0");
    require(to != 0, "Invalid to address");
@>  require(to_handler != 0, "Invalid to handler address");
    require(to_token != 0, "Invalid to token address");

    if (mode == SettlementMode.MintBurn) {
        _erc20_lock(msg.sender, address(this), amount);
    } else if (mode == SettlementMode.LockUnlock) {
        _erc20_lock(msg.sender, address(this), amount);
    } else if (mode == SettlementMode.LockMint) {
        _erc20_lock(msg.sender, address(this), amount);
    } else if (mode == SettlementMode.BurnUnlock) {
        _erc20_burn(msg.sender, amount);
    }
    ...
}
// Settlement.receive_cross_chain_msg
function receive_cross_chain_msg(
    uint256 txid,
    string memory from_chain,
    uint256 from_address,
    uint256 from_handler,
    address to_handler,
    PayloadType payload_type,
    bytes calldata payload,
    uint8 sign_type, // validators signature type /  multisig or bls sr25519
    bytes calldata signatures // signature array
) external {
    ...

@>  bool result = ISettlementHandler(to_handler).receive_cross_chain_msg( // this will be reverted
        txid,
        from_chain,
        from_address,
        from_handler,
        payload_type,
        payload,
        sign_type,
        signatures
    );
  
    CrossChainMsgStatus status = CrossChainMsgStatus.Failed;
    if (result == true) {
        status = CrossChainMsgStatus.Success;
        receive_cross_txs[txid].status = CrossChainMsgStatus.Success;
    } else {
@>      receive_cross_txs[txid].status = CrossChainMsgStatus.Failed;
    }

    emit CrossChainHandleResult(
        txid,
@>      status,
        contract_chain_name,
        from_chain,
        address(to_handler),
        from_handler,
        payload_type
    );
}
function receive_cross_chain_msg(
    uint256 /**txid */,
    string memory from_chain,
    uint256 /**from_address */,
    uint256 from_handler,
    PayloadType payload_type,
    bytes calldata payload,
    uint8 /**sign type */,
    bytes calldata /**signaturs */
) external onlySettlement returns (bool) {
    //  from_handler need in whitelist
@>  if (is_valid_handler(from_chain, from_handler) == false) {
@>      return false;
    }
    ...
}
function receive_cross_chain_callback(
    uint256 txid,
    string memory from_chain,
    uint256 from_handler,
    CrossChainMsgStatus status,
    uint8 /* sign_type */, // validators signature type /  multisig or bls sr25519
    bytes calldata /* signatures */
) external onlySettlement returns (bool) {
    //  from_handler need in whitelist
@>  if (is_valid_handler(from_chain, from_handler) == false) {
@>      return false;
    }

    require(
        create_cross_txs[txid].status == CrossChainTxStatus.Pending,
        "invalid CrossChainTxStatus"
    );

    if (status == CrossChainMsgStatus.Success) {
        if (mode == SettlementMode.MintBurn) {
            _erc20_burn(address(this), create_cross_txs[txid].amount);
        }

        create_cross_txs[txid].status = CrossChainTxStatus.Settled;
    }

    if (status == CrossChainMsgStatus.Failed) {
        create_cross_txs[txid].status = CrossChainTxStatus.Failed;
    }

    return true;
}
function cross_chain_erc20_settlement(
    string memory to_chain,
    uint256 to_handler,
    uint256 to_token,
    uint256 to,
    uint256 amount
) external {
    require(amount > 0, "Amount must be greater than 0");
    require(to != 0, "Invalid to address");
    require(to_handler != 0, "Invalid to handler address");
    require(to_token != 0, "Invalid to token address");
+   require(is_valid_handler(to_chain, to_handler), "not valid destination");

    if (mode == SettlementMode.MintBurn) {
        _erc20_lock(msg.sender, address(this), amount);
    } else if (mode == SettlementMode.LockUnlock) {
        _erc20_lock(msg.sender, address(this), amount);
    } else if (mode == SettlementMode.LockMint) {
        _erc20_lock(msg.sender, address(this), amount);
    } else if (mode == SettlementMode.BurnUnlock) {
        _erc20_burn(msg.sender, amount);
    }
    ...
}
