@Test(expected = IllegalArgumentException.class)
    public void shouldFailAddVWithNullVertexLabelTraversal() {
        g.addV((Traversal<?, String>) null);
    }
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-core/src/test/java/org/apache/tinkerpop/gremlin/process/traversal/dsl/graph/GraphTraversalSourceTest.java