问Tinkerpop控制台读取带有subgraph的graphml时返回nullpointerexception

EN

gremlin> graph = TinkerGraph.open()
==>tinkergraph[vertices:0 edges:0]
gremlin> graph.io(graphml()).readGraph('data/xsmall0.graphml')
==>null
gremlin> g = graph.traversal()
==>graphtraversalsource[tinkergraph[vertices:3 edges:2], standard]
gremlin> g.V()
==>v[n1]
==>v[n2]
==>v[n3]
gremlin> graph = TinkerGraph.open()
==>tinkergraph[vertices:0 edges:0]
gremlin> graph.io(graphml()).readGraph('data/xsmall2.graphml')
java.lang.NullPointerException
Type ':help' or ':h' for help.
Display stack trace? [yN]y
java.lang.NullPointerException
    at org.apache.tinkerpop.gremlin.structure.io.graphml.GraphMLReader.readGraph(GraphMLReader.java:175)
    at org.apache.tinkerpop.gremlin.structure.io.graphml.GraphMLIo.readGraph(GraphMLIo.java:92)
    at org.apache.tinkerpop.gremlin.structure.io.Io$readGraph.call(Unknown Source)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:125)
    at groovysh_evaluate.run(groovysh_evaluate:3)
    at org.codehaus.groovy.vmplugin.v7.IndyInterface.selectMethod(IndyInterface.java:232)
    at org.codehaus.groovy.tools.shell.Interpreter.evaluate(Interpreter.groovy:70)
    at org.codehaus.groovy.tools.shell.Groovysh.execute(Groovysh.groovy:191)
    at org.apache.tinkerpop.gremlin.console.GremlinGroovysh.super$3$execute(GremlinGroovysh.groovy)
    at sun.reflect.GeneratedMethodAccessor14.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:93)
    at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:325)
    at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1213)
    at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.invokeMethodOnSuperN(ScriptBytecodeAdapter.java:132)
    at org.apache.tinkerpop.gremlin.console.GremlinGroovysh.execute(GremlinGroovysh.groovy:72)
    at org.codehaus.groovy.tools.shell.Shell.leftShift(Shell.groovy:122)
    at org.codehaus.groovy.tools.shell.ShellRunner.work(ShellRunner.groovy:95)
    at org.codehaus.groovy.tools.shell.InteractiveShellRunner.super$2$work(InteractiveShellRunner.groovy)
    at sun.reflect.GeneratedMethodAccessor34.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:93)
    at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:325)
    at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1213)
    at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.invokeMethodOnSuperN(ScriptBytecodeAdapter.java:132)
    at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.invokeMethodOnSuper0(ScriptBytecodeAdapter.java:152)
    at org.codehaus.groovy.tools.shell.InteractiveShellRunner.work(InteractiveShellRunner.groovy:124)
    at org.codehaus.groovy.tools.shell.ShellRunner.run(ShellRunner.groovy:59)
    at org.codehaus.groovy.tools.shell.InteractiveShellRunner.super$2$run(InteractiveShellRunner.groovy)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:93)
    at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:325)
    at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1213)
    at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.invokeMethodOnSuperN(ScriptBytecodeAdapter.java:132)
    at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.invokeMethodOnSuper0(ScriptBytecodeAdapter.java:152)
    at org.codehaus.groovy.tools.shell.InteractiveShellRunner.run(InteractiveShellRunner.groovy:83)
    at org.codehaus.groovy.vmplugin.v7.IndyInterface.selectMethod(IndyInterface.java:232)
    at org.apache.tinkerpop.gremlin.console.Console.<init>(Console.groovy:143)
    at org.codehaus.groovy.vmplugin.v7.IndyInterface.selectMethod(IndyInterface.java:232)
    at org.apache.tinkerpop.gremlin.console.Console.main(Console.groovy:450)
gremlin> 

<?xml version="1.0" encoding="UTF-8"?>
<graphml>
  <graph id="G" edgedefault="directed">
    <node id="n0"/>
    <node id="n1"/>
    <node id="n2">
      <graph id="n2::" edgedefault="directed">
        <node id="n2::n0"/>
        <node id="n2::n1">
          <graph id="n2::n1::" edgedefault="directed">
            <node id="n2::n1::n0"/>
            <node id="n2::n1::n1"/>
            <node id="n2::n1::n2"/>
            <edge id="e0" source="n2::n1::n1" target="n2::n1::n0"/>
            <edge id="e1" source="n2::n1::n2" target="n2::n1::n1"/>
          </graph>
        </node>
        <node id="n2::n2"/>
        <edge id="e2" source="n2::n1" target="n2::n0"/>
        <edge id="e3" source="n2::n2" target="n2::n1"/>
      </graph>
    </node>
    <edge id="e4" source="n1" target="n0"/>
    <edge id="e5" source="n1" target="n2::n1"/>
    <edge id="e6" source="n0" target="n2"/>
  </graph>
</graphml>