Reported here: https://github.com/bitcoin/bitcoin/pull/10244#issuecomment-378035305

Travis log here: https://travis-ci.org/bitcoin/bitcoin/jobs/361243719#L2810

Note the JSON exception:

```
test_framework.authproxy.JSONRPCException: The mempool was not loaded yet (-1) 
```

That exception was introduced here: https://github.com/bitcoin/bitcoin/commit/cb1e319fe9e198c9c5cf5236fe9af5a3d748b9e8#diff-a0c8f511d90e83aa9b5857e819ced344R1617

cc @jtimon 
{
    if(pindex) {
        std::lock_guard<std::mutex> lock(cs_blockchange);
        latestblock.hash = pindex->GetBlockHash();
        latestblock.height = pindex->nHeight;
    }
    cond_blockchange.notify_all();
}
static UniValue waitfornewblock(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() > 1)
        throw std::runtime_error(
            RPCHelpMan{"waitfornewblock",
                "\nWaits for a specific new block and returns useful info about it.\n"
                "\nReturns the current block on timeout or exit.\n",
                {
                    {"timeout", RPCArg::Type::NUM,  "0", "Time in milliseconds to wait for a response. 0 indicates no timeout."},
                },
                RPCResult{
            "{                           (json object)\n"
            "  \"hash\" : {       (string) The blockhash\n"
            "  \"height\" : {     (int) Block height\n"
            "}\n"
                },
                RPCExamples{
                    HelpExampleCli("waitfornewblock", "1000")
            + HelpExampleRpc("waitfornewblock", "1000")
                },