title: NullPointerException in ProtoBuf when Kryo serialization is used with Spark
body: <p>I am getting the following error in my spark application when it is trying to serialize a protobuf field which is a map of key String and value float. Kryo serialization is being used in the spark app.</p>
  
  <pre><code>Caused by: java.lang.NullPointerException
      at com.google.protobuf.UnmodifiableLazyStringList.size(UnmodifiableLazyStringList.java:68)
      at java.util.AbstractList.add(AbstractList.java:108)
      at com.esotericsoftware.kryo.serializers.CollectionSerializer.read(CollectionSerializer.java:134)
      at com.esotericsoftware.kryo.serializers.CollectionSerializer.read(CollectionSerializer.java:40)
      at com.esotericsoftware.kryo.Kryo.readObject(Kryo.java:731)
      at com.esotericsoftware.kryo.serializers.ObjectField.read(ObjectField.java:125)
      ... 71 more
  </code></pre>
  
  <p>Has anyone faced this issue before? Is there a way to resolve it?</p>