ERC721Airdrop constructor
Implementation
ERC721Airdrop(String contractAddress, String providerUrl,
[String? signerPrivateKey]) {
_provider = JsonRpcProvider(providerUrl);
_signer =
signerPrivateKey == null ? null : Wallet(signerPrivateKey, _provider);
_contract = Contract(contractAddress, _abi, _signer ?? _provider);
}