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>();
    for (const auto& coin_pair : coins) {
        std::unique_ptr<FlatSigningProvider> coin_keys = GetSigningProvider(coin_pair.second.out.scriptPubKey, true);
        if (!coin_keys) {
            continue;
        }
        *keys = Merge(*keys, *coin_keys);
    }
    return ::SignTransaction(tx, keys.get(), coins, sighash, input_errors);
}
SigningResult DescriptorScriptPubKeyMan::SignMessage(const std::string& message, const PKHash& pkhash, std::string& str_sig) const
{
    std::unique_ptr<FlatSigningProvider> keys = GetSigningProvider(GetScriptForDestination(pkhash), true);
    if (!keys) {
        return SigningResult::PRIVATE_KEY_NOT_AVAILABLE;
    }
    CKey key;
    if (!keys->GetKey(ToKeyID(pkhash), key)) {
        return SigningResult::PRIVATE_KEY_NOT_AVAILABLE;
    }
    if (!MessageSign(key, message, str_sig)) {
        return SigningResult::SIGNING_FAILED;
    }
    return SigningResult::OK;
}
TransactionError DescriptorScriptPubKeyMan::FillPSBT(PartiallySignedTransaction& psbtx, const PrecomputedTransactionData& txdata, int sighash_type, bool sign, bool bip32derivs, int* n_signed, bool finalize) const