I guess this has something to do with race conditions. Although I couldn't reproduce the exact issue, I found a similar race condition that caused an NPE in `expireSchemaElement` method (https://github.com/JanusGraph/janusgraph/issues/2898).  The fix is here: https://github.com/JanusGraph/janusgraph/pull/2899 which will be released in the next minor version (0.6.1). This PR also includes the temporary fix proposed by Sergey:

public InternalVertex getInternalVertex(long vertexId) {
    // TODO temporary fix
    if (isClosed()) {
        return null;
    }
    //return vertex but potentially check for existence
    return vertexCache.get(vertexId, internalVertexRetriever);
}

If anyone is able to find a steady way to reproduce this issue and/or encounter a similar NPE issue somewhere else, please let me know, thanks! A code review is also very welcome.

Best,
Boxuan