@Test(expected = IllegalArgumentException.class)
    public void shouldNotCreateVertexIndexWithNullKey() {
        final TinkerGraph g = TinkerGraph.open();
        g.createIndex(null, Vertex.class);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Stephen Mallette (http://stephen.genoprime.com)
tinkergraph-gremlin/src/test/java/org/apache/tinkerpop/gremlin/tinkergraph/structure/TinkerGraphTest.java