Running master 4d661ba:
```
src/bitcoin-cli combinepsbt [""] or src/bitcoin-cli combinepsbt []
```

```
  * frame #0: 0x000000010016054a bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional_detail::optional_base<CMutableTransaction>::is_initialized(this=0x0000000000000000) const at optional.hpp:394 [opt]
    frame #1: 0x000000010016054a bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional_detail::optional_base<CMutableTransaction>::optional_base(this=<unavailable>, rhs=0x0000000000000000) at optional.hpp:196 [opt]
    frame #2: 0x0000000100160542 bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional<CMutableTransaction>::optional(this=0x000070000b9fe7c0, (null)=0x0000000000000000) at optional.hpp:958 [opt]
    frame #3: 0x0000000100160542 bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional<CMutableTransaction>::optional(this=0x000070000b9fe7c0, (null)=0x0000000000000000) at optional.hpp:958 [opt]
    frame #4: 0x0000000100160542 bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(this=0x000070000b9fe7c0, psbt_in=0x0000000000000000) at sign.h:576 [opt]
    frame #5: 0x0000000100142ae5 bitcoind`combinepsbt(JSONRPCRequest const&) [inlined] PartiallySignedTransaction::PartiallySignedTransaction(this=0x000070000b9fe700, psbt_in=<unavailable>) at sign.h:576 [opt]
    frame #6: 0x0000000100142ae0 bitcoind`combinepsbt(request=<unavailable>) at rawtransaction.cpp:1526 [opt]
    frame #7: 0x0000000100170f0a bitcoind`CRPCTable::execute(this=<unavailable>, request=0x000070000b9fead0) const at server.cpp:557 [opt]
```
{
    
    CCoinsView viewDummy;
    CCoinsViewCache view(&viewDummy);
    {
        LOCK2(cs_main, mempool.cs);
        CCoinsViewCache &viewChain = *pcoinsTip;
        CCoinsViewMemPool viewMempool(&viewChain, mempool);
        view.SetBackend(viewMempool); 
        for (const CTxIn& txin : mtx.vin) {
            view.AccessCoin(txin.prevout); 
        }
        view.SetBackend(viewDummy); 
    }
    
    if (!prevTxsUnival.isNull()) {
        UniValue prevTxs = prevTxsUnival.get_array();
        for (unsigned int idx = 0; idx < prevTxs.size(); ++idx) {
            const UniValue& p = prevTxs[idx];
            if (!p.isObject()) {
                throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "expected object with {\"txid'\",\"vout\",\"scriptPubKey\"}");
            }
            UniValue prevOut = p.get_obj();
            RPCTypeCheckObj(prevOut,
                {
                    {"txid", UniValueType(UniValue::VSTR)},
                    {"vout", UniValueType(UniValue::VNUM)},
                    {"scriptPubKey", UniValueType(UniValue::VSTR)},