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
}
```
{
    AssertLockHeld(cs_desc_man);
    
    std::unique_ptr<FlatSigningProvider> out_keys = std::make_unique<FlatSigningProvider>();
    std::vector<CScript> scripts_temp;
    if (!m_wallet_descriptor.descriptor->ExpandFromCache(index, m_wallet_descriptor.cache, scripts_temp, *out_keys)) return nullptr;
    if (HavePrivateKeys() && include_private) {
        FlatSigningProvider master_provider;
        master_provider.keys = GetKeys();
        m_wallet_descriptor.descriptor->ExpandPrivate(index, master_provider, *out_keys);
    }
    return out_keys;
}
std::unique_ptr<SigningProvider> DescriptorScriptPubKeyMan::GetSolvingProvider(const CScript& script) const
{
    return GetSigningProvider(script, false);
}
bool DescriptorScriptPubKeyMan::CanProvide(const CScript& script, SignatureData& sigdata)
{
    return IsMine(script);
}
bool DescriptorScriptPubKeyMan::SignTransaction(CMutableTransaction& tx, const std::map<COutPoint, Coin>& coins, int sighash, std::map<int, bilingual_str>& input_errors) const