Unreachable code. Unnamed return variable can remain unassigned.のエラーを解消したいです

Unreachable code. Unnamed return variable can remain unassigned.

ソースコードを入力

pragma solidity >=0.7.0 <0.9.0;

import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
import "./Base64.sol";

contract OnChainnft is ERC721Enumerable, Ownable {
  using Strings for uint256;

  string public baseExtension = ".json";
  uint256 public cost = 0.05 ether;
  uint256 public maxSupply = 10000;

  constructor() ERC721("On Chain NFT", "OCN") {}

  // public
  function mint(uint256 _mintAmount) public payable {
    uint256 supply = totalSupply();
    require(_mintAmount > 0);
    require(supply + _mintAmount <= maxSupply);

    if (msg.sender != owner()) {
      require(msg.value >= cost * _mintAmount);
    }

    for (uint256 i = 1; i <= _mintAmount; i++) {
      _safeMint(msg.sender, supply + i);
    }
  }

  function buildImage() public pure returns(string memory) {
    return Base64.encode(bytes(abi.encodePacked(
        '<svg version="1.0" xmlns="http://www.w3.org/2000/svg">',
        '<width="300.000000pt" height="300.000000pt" viewBox="0 0 300.000000 300.000000">',
        '<preserveAspectRatio="xMidYMid meet">',
        '<path d="M1015 1830 c6 -10 3 -12 -14 -7 -44 11 -53 -15 -51 -147 3 -166 3 -161 -16 -146 -27 23 -37 50 -23 66 8 10 8 14 0 14 -6 0 -11 -5 -11 -11 0 -5 -4 -7 -10 -4 -5 3 -10 14 -10 23 0 9 -14 30 -30 46 -17 16 -30 35 -30 42 0 7 -6 17 -14 21 -14 9 -41 54 -38 65 5 20 -37 33 -107 33 -122 0 -111 33 -111 -329 0 -213 3 -317 11 -321 6 -4 8 -14 5 -22 -4 -9 -2 -11 7 -6 6 4 44 6 83 4 53 -2 73 1 83 12 8 10 12 62 13 161 1 80 4 143 8 141 4 -2 15 -22 25 -45 10 -22 22 -40 27 -40 5 0 16 -16 24 -35 8 -19 18 -35 23 -35 5 0 12 -7 15 -16 3 -8 2 -12 -3 -9 -5 3 -13 1 -16 -5 -4 -6 -3 -10 2 -9 25 4 34 -3 29 -23 -3 -13 -3 -17 0 -10 11 24 25 12 20 -18 -2 -17 0 -28 5 -24 5 3 7 11 3 17 -5 10 5 0 45 -49 10 -12 176 -12 187 -1 9 9 4 641 -6 651 -5 5 -31 10 -59 12 -28 1 -56 6 -62 10 -8 5 -9 3 -4 -6z m-275 -39 c0 -6 50 -85 112 -178 61 -92 113 -176 116 -185 9 -38 17 -15 15 44 -2 35 -4 123 -5 196 l-3 132 80 0 80 0 -3 -312 -3 -313 -80 2 -80 2 -104 158 c-58 87 -113 176 -123 197 -17 39 -18 38 -14 -160 l4 -199 -78 0 -79 0 0 312 0 313 83 0 c45 0 82 -4 82 -9z"/>',
        '<path d="M1819 1824 c-8 -10 -15 -48 -17 -90 -3 -88 4 -95 93 -88 71 6 117 -2 94 -17 -12 -7 -12 -9 2 -9 13 0 16 -18 18 -116 1 -75 -2 -120 -9 -129 -7 -8 -8 -20 -4 -27 11 -18 14 -132 3 -138 -4 -3 -2 -18 5 -32 8 -15 11 -30 8 -34 -4 -4 -1 -4 7 1 7 4 47 7 88 7 85 -1 109 13 106 63 -6 78 -5 354 1 363 3 6 2 13 -3 16 -10 7 -9 25 2 43 8 13 121 13 134 0 11 -12 21 -8 43 15 29 31 29 139 0 168 -12 12 -26 18 -32 14 -12 -8 -423 -8 -484 0 -31 4 -45 1 -55 -10z m571 -90 l0 -72 -106 6 -106 5 4 -249 3 -249 -80 0 -80 0 3 190 c1 105 3 217 5 251 2 50 -1 60 -13 58 -8 -2 -56 -5 -106 -8 l-91 -5 -1 72 -1 72 285 0 284 0 0 -71z"/>',
        '<path d="M1292 1821 l-24 -6 -2 -322 -1 -323 25 -9 c32 -13 137 -13 160 -1 18 10 33 47 22 54 -4 2 -5 44 -2 93 l5 88 105 -2 c67 -2 113 1 128 9 21 11 23 17 20 82 -3 67 -20 110 -41 99 -20 -11 -67 -11 -67 -1 0 9 -3 9 -12 0 -7 -7 -38 -12 -70 -12 -64 0 -75 10 -64 54 6 24 6 24 133 22 161 -3 163 -2 156 97 -3 40 -11 75 -17 79 -13 9 -419 7 -454 -1z m448 -86 l0 -70 -150 3 -150 3 0 -60 0 -61 133 0 132 0 0 -67 0 -68 -134 4 -134 3 5 -123 6 -124 -82 0 -81 0 0 315 0 315 228 0 227 0 0 -70z"/>',
        '<path d="M1708 1623 c7 -3 16 -2 19 1 4 3 -2 6 -13 5 -11 0 -14 -3 -6 -6z"/>',
        '</svg>'
    )));

     return string(abi.encodePacked(
        'data:application/json;base64,', Base64.encode(bytes(abi.encodePacked(
            '{"name": "OnChainTest',
            '", "description": "First test',
            '", "image": "data:image/svg+xml;base64,',
            buildImage(),
            '"}'
        )))
    ));
    }

  function walletOfOwner(address _owner) public view returns (uint256[] memory) {
    uint256 ownerTokenCount = balanceOf(_owner);
    uint256[] memory tokenIds = new uint256[](ownerTokenCount);
    for (uint256 i; i < ownerTokenCount; i++) {
      tokenIds[i] = tokenOfOwnerByIndex(_owner, i);
    }
    return tokenIds;
  }

  function tokenURI(uint256 tokenId)
    public
    view
    virtual
    override
    returns (string memory)
  {
    require(
      _exists(tokenId),
      "ERC721Metadata: URI query for nonexistent token"
    );

  }

  function setCost(uint256 _newCost) public onlyOwner {
    cost = _newCost;
  }

  function setBaseExtension(string memory _newBaseExtension) public onlyOwner {
    baseExtension = _newBaseExtension;
  }

  function withdraw() public payable onlyOwner {
    // This will pay HashLips 5% of the initial sale.
    // You can remove this if you want, or keep it in to support HashLips and his channel.
    // =============================================================================
    (bool hs, ) = payable(0x943590A42C27D08e3744202c4Ae5eD55c2dE240D).call{value: address(this).balance * 5 / 100}("");
    require(hs);
    // =============================================================================

    // This will payout the owner 95% of the contract balance.
    // Do not remove this otherwise you will not be able to withdraw the funds.
    // =============================================================================
    (bool os, ) = payable(owner()).call{value: address(this).balance}("");
    require(os);
    // =============================================================================
  }
}