`ScanForWalletTransactions()` has the following comment about its return value:

```
 * Returns null if scan was successful. Otherwise, if a complete rescan was not
 * possible (due to pruning or corruption), returns pointer to the most recent
 * block that could not be scanned.
```

This is not strictly true. If `abortrescan` is called, then `ScanForWalletTransactions()` will return `nullptr`, even though the scan wasn't successful to the tip.

This in turn causes a minor bug in `importmulti`, where the call doesn't return the correct error and warning about potentially not finding transactions if `abortrescan` was called.

I think the correct fix is for `ScanForWalletTransactions()` to inform the caller that the rescan was aborted.
std::vector<CKeyID> vAffected;
    for (const auto& entry : mapWallet) {
        
        const CWalletTx &wtx = entry.second;
        CBlockIndex* pindex = LookupBlockIndex(wtx.hashBlock);
        if (pindex && chainActive.Contains(pindex)) {
            
            int nHeight = pindex->nHeight;
            for (const CTxOut &txout : wtx.tx->vout) {
                
                CAffectedKeysVisitor(*this, vAffected).Process(txout.scriptPubKey);
                for (const CKeyID &keyid : vAffected) {
                    
                    std::map<CKeyID, CBlockIndex*>::iterator rit = mapKeyFirstBlock.find(keyid);
                    if (rit != mapKeyFirstBlock.end() && nHeight < rit->second->nHeight)
                        rit->second = pindex;
                }
                vAffected.clear();
            }
        }
    }
    
    for (const auto& entry : mapKeyFirstBlock)
        mapKeyBirth[entry.first] = entry.second->GetBlockTime() - TIMESTAMP_WINDOW; 
}
/**
 * Compute smart timestamp for a transaction being added to the wallet.
 *
 * Logic:
 * - If sending a transaction, assign its timestamp to the current time.