VM Exception while processing transaction: revert - blockchain

Question

I have 2 Contracts. One is an ERC721 Token (NFTCollectables). The other one is a market including an auction system (NFTMarket).
An auction can be claimed after it ended and only by the highest bidder.
When claiming an auction the transfer method of the NFTCollectables contract is called to transfer the NFT from the markets address to the address of the highest bidder.
I do not exactly understand why the exception comes, but it occurs at/inside the transfer method of the NFTCollectables contract. The strange thing is that even the last line of code inside the transfer method is getting executed (tested by putting a require(false, 'test') after _transfer(msg.sender, to, nftId)). But nothing after ctr.transfer(auction.highestBid.bidder, auction.nftId) is getting executed (tested by putting a require(false, 'test') after it).
Could it have to do with the gas limit?
Any idea is appreciated, thanks!
NFTMarket
function claimAuction(uint auctionIndex) external {
require(auctionIndex < auctions.length, "no auction");
Auction memory auction = auctions[auctionIndex];
require(block.timestamp <= auction.end, "auction still active");
NFTCollectables ctr = NFTCollectables(nftCollectablesAddress);
ctr.transfer(auction.highestBid.bidder, auction.nftId);
// deleting auction from active auctions list
for (uint i; i < activeAuctionIndexes.length; i++) {
if (activeAuctionIndexes[i] == auctionIndex) {
delete activeAuctionIndexes[i];
break;
}
}
emit AuctionEnd(auction.highestBid.bidder, auction.highestBid.price, auction.nftId);
}
NFTCollectables
function transfer(address payable to, uint nftId) external payable {
require(_exists(nftId), "transfer of non existing token");
require(_isApprovedOrOwner(msg.sender, nftId), "Sender not approved nor owner");
_transfer(msg.sender, to, nftId);
}

Answer 1

If you have deployed the contracts on the public network, like mainnet or testnet, use https://tenderly.co or EtherScan to debug the revert reason.
If you are running the contracts using unit tests then keep modifying the contracts e.g. by removing lines to see when it starts failing. Or alternatively use a better smart contract development framework like Brownie which will give you the cause of revert right away.

Answer 2

We had implemented a function on our codebase that i can share with you. This function is used to transfer ownership of nft to who wins the auction.
function transferNFTtoNewOwner(NFTItem memory t,address oldOwner, address newOwner) internal {
require(newOwner != address(0), "New owner can't be address zero.");
XXXX storage r = creatureList[t.tokenAddress][t.tokenId];
IERC721 nft = IERC721(t.tokenAddress);
nft.safeTransferFrom(oldOwner, newOwner, t.tokenId);
address currOwner = nft.ownerOf(t.tokenId);
require(newOwner == currOwner, "Problem on nft transfer");
r.owner = newOwner;
}
The major differences are we used safetransferfrom here. If your contract owns the NFT then you can call safeTransfer. and after that we checked the nft owner with require statement. If you put this statement and change your transfer function to safetransfer and the transaction still revert without giving any error on etherscan then you can investigate about your nft contract.