java.lang.ClassCastException: scala.None$ cannot be cast to java.lang.Double

Cast Error when transforming leapframe.
I'm using the imputer from mleap-spark-extension and some fields of my data are null. I init these structs as `ScalarType.Double.setNullable(true)`. But when I use the model to transform my generated leapframe, there comes the error.
```
java.lang.ClassCastException: scala.None$ cannot be cast to java.lang.Double
at ml.combust.mleap.runtime.transformer.feature.Imputer$$anonfun$1.apply(Imputer.scala:14)
at ml.combust.mleap.runtime.frame.Row$class.udfValue(Row.scala:241)
at ml.combust.mleap.runtime.frame.ArrayRow.udfValue(ArrayRow.scala:17)
at ml.combust.mleap.runtime.frame.Row$class.withValue(Row.scala:221)
at ml.combust.mleap.runtime.frame.ArrayRow.withValue(ArrayRow.scala:17)
at ml.combust.mleap.runtime.frame.DefaultLeapFrame$$anonfun$withColumn$1$$anonfun$apply$2$$anonfun$2.apply(DefaultLeapFrame.scala:54)
at ml.combust.mleap.runtime.frame.DefaultLeapFrame$$anonfun$withColumn$1$$anonfun$apply$2$$anonfun$2.apply(DefaultLeapFrame.scala:54)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
at scala.collection.immutable.List.foreach(List.scala:392)
at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:35)
at scala.collection.mutable.ListBuffer.foreach(ListBuffer.scala:45)
at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
at scala.collection.AbstractTraversable.map(Traversable.scala:104)
at ml.combust.mleap.runtime.frame.DefaultLeapFrame$$anonfun$withColumn$1$$anonfun$apply$2.apply(DefaultLeapFrame.scala:54)
at ml.combust.mleap.runtime.frame.DefaultLeapFrame$$anonfun$withColumn$1$$anonfun$apply$2.apply(DefaultLeapFrame.scala:53)
at scala.util.Success$$anonfun$map$1.apply(Try.scala:237)
at scala.util.Try$.apply(Try.scala:192)
at scala.util.Success.map(Try.scala:237)
at ml.combust.mleap.runtime.frame.DefaultLeapFrame$$anonfun$withColumn$1.apply(DefaultLeapFrame.scala:52)
at ml.combust.mleap.runtime.frame.DefaultLeapFrame$$anonfun$withColumn$1.apply(DefaultLeapFrame.scala:49)
at scala.util.Success.flatMap(Try.scala:231)
at ml.combust.mleap.runtime.frame.DefaultLeapFrame.withColumn(DefaultLeapFrame.scala:48)
at ml.combust.mleap.runtime.frame.SimpleTransformer$class.transform(Transformer.scala:112)
at ml.combust.mleap.runtime.transformer.feature.Imputer.transform(Imputer.scala:11)
at ml.combust.mleap.runtime.transformer.Pipeline$$anonfun$transform$2$$anonfun$apply$1.apply(Pipeline.scala:28)
at ml.combust.mleap.runtime.transformer.Pipeline$$anonfun$transform$2$$anonfun$apply$1.apply(Pipeline.scala:28)
at scala.util.Success.flatMap(Try.scala:231)
at ml.combust.mleap.runtime.transformer.Pipeline$$anonfun$transform$2.apply(Pipeline.scala:28)
at ml.combust.mleap.runtime.transformer.Pipeline$$anonfun$transform$2.apply(Pipeline.scala:28)
at scala.collection.LinearSeqOptimized$class.foldLeft(LinearSeqOptimized.scala:124)
at scala.collection.immutable.List.foldLeft(List.scala:84)
at ml.combust.mleap.runtime.transformer.Pipeline.transform(Pipeline.scala:28)
at com.zhihu.saturn.service.impl.RecommendationImpl.getSlotCardRank(RecommendationImpl.scala:31)
at com.zhihu.saturn.thrift.RecommendationService$Processor$get_slot_card_rank.getResult(RecommendationService.java:181)
at com.zhihu.saturn.thrift.RecommendationService$Processor$get_slot_card_rank.getResult(RecommendationService.java:165)
at org.apache.thrift.ProcessFunction.process(ProcessFunction.java:39)
at com.zhihu.tzone.server.TProcessorInvocationHandler.invoke(TProcessorInvocationHandler.java:55)
at com.sun.proxy.$Proxy19.process(Unknown Source)
at org.apache.thrift.TMultiplexedProcessor.process(TMultiplexedProcessor.java:123)
at com.zhihu.tzone.server.TZoneServlet.doPost(TZoneServlet.java:98)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:845)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:583)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1160)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:511)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1092)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:119)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:134)
at org.eclipse.jetty.server.Server.handle(Server.java:518)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:308)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:244)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:273)
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:95)
at org.eclipse.jetty.io.SelectChannelEndPoint$2.run(SelectChannelEndPoint.java:93)
at org.eclipse.jetty.util.thread.strategy.ExecuteProduceConsume.produceAndRun(ExecuteProduceConsume.java:246)
at org.eclipse.jetty.util.thread.strategy.ExecuteProduceConsume.run(ExecuteProduceConsume.java:156)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:654)
at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:572)
at java.lang.Thread.run(Thread.java:745)
```
Some column values are null as showed below.
```
null| 0.0| null| 0.0| 1.0| 1.0| 1.0| 1.0| 1.0| 1.0|
```

Is the imputer cannot handle none value? I suppose imputer is designed to fill null values.