I have 2 legacy wallets and on both if I run `upgradewallet` it will cause bitcoin core to crash. This should just return me an error string

Currenl result from `getwalletinfo`:
```
{
  "walletname": "Wallet",
  "walletversion": 169900,
  "format": "bdb",
  "balance": 0.00000000,
  "unconfirmed_balance": 0.00000000,
  "immature_balance": 0.00000000,
  "txcount": 80,
  "keypoololdest": 1613078825,
  "keypoolsize": 941,
  "keypoolsize_hd_internal": 941,
  "paytxfee": 0.00000000,
  "private_keys_enabled": false,
  "avoid_reuse": true,
  "scanning": false,
  "descriptors": false
}
```
std::unique_ptr<FlatSigningProvider> keys = std::make_unique<FlatSigningProvider>();
        std::unique_ptr<FlatSigningProvider> script_keys = GetSigningProvider(script, sign);
        if (script_keys) {
            *keys = Merge(*keys, *script_keys);
        } else {
            
            script_keys = std::make_unique<FlatSigningProvider>();
            for (const auto& pk_pair : input.hd_keypaths) {
                const CPubKey& pubkey = pk_pair.first;
                std::unique_ptr<FlatSigningProvider> pk_keys = GetSigningProvider(pubkey);
                if (pk_keys) {
                    *keys = Merge(*keys, *pk_keys);
                }
            }
        }
        SignPSBTInput(HidingSigningProvider(keys.get(), !sign, !bip32derivs), psbtx, i, &txdata, sighash_type, nullptr, finalize);
        bool signed_one = PSBTInputSigned(input);
        if (n_signed && (signed_one || !sign)) {
            
            
            
            (*n_signed)++;
        }
    }
    
    for (unsigned int i = 0; i < psbtx.tx->vout.size(); ++i) {
        std::unique_ptr<SigningProvider> keys = GetSolvingProvider(psbtx.tx->vout.at(i).scriptPubKey);
        if (!keys) {
            continue;
        }