function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata) external override returns (bytes4) {
      _addNft(msg.sender, tokenId);

function _addNft(address nftContract, uint256 tokenId) internal {
    nfts.push(
    Nft({
        tokenId: tokenId,
        nftContract: nftContract
    })
    );
