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 
#include <rpc/util.h>
#include <script/descriptor.h>
#include <streams.h>
#include <sync.h>
#include <txdb.h>
#include <txmempool.h>
#include <util/strencodings.h>
#include <util/system.h>
#include <validation.h>
#include <validationinterface.h>
#include <versionbitsinfo.h>
#include <warnings.h>
#include <assert.h>
#include <stdint.h>
#include <univalue.h>
#include <boost/thread/thread.hpp> 
#include <memory>
#include <mutex>
#include <condition_variable>
struct CUpdatedBlock
{
    uint256 hash;
    int height;
};
static Mutex cs_blockchange;
static std::condition_variable cond_blockchange;
static CUpdatedBlock latestblock;
/* Calculate the difficulty for a given block index.
 */
double GetDifficulty(const CBlockIndex* blockindex)