How to reproduce

• Open history tool
• Open sitemap tool
• Create a new page with a new content "Article"

  Caused by: java.lang.IllegalArgumentException: The object can not be converted to json string
  	at org.ametys.core.util.JSONUtils.convertObjectToJson(JSONUtils.java:194)
  	at org.ametys.core.cocoon.JSonReader.generate(JSonReader.java:61)
  	at org.apache.cocoon.components.pipeline.impl.AbstractCachingProcessingPipeline.processReader(AbstractCachingProcessingPipeline.java:921)
  	... 70 more
  Caused by: org.codehaus.jackson.map.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: java.util.HashMap["workflow"]->java.util.ArrayList[0]->java.util.HashMap["actionLabel"])
  	at org.codehaus.jackson.map.JsonMappingException.wrapWithPath(JsonMappingException.java:218)
  	at org.codehaus.jackson.map.JsonMappingException.wrapWithPath(JsonMappingException.java:183)
  	at org.codehaus.jackson.map.ser.std.SerializerBase.wrapAndThrow(SerializerBase.java:140)
  	at org.codehaus.jackson.map.ser.std.MapSerializer.serializeFields(MapSerializer.java:266)
  	at org.codehaus.jackson.map.ser.std.MapSerializer.serialize(MapSerializer.java:186)
  	at org.codehaus.jackson.map.ser.std.MapSerializer.serialize(MapSerializer.java:23)
  	at org.codehaus.jackson.map.ser.std.StdContainerSerializers$IndexedListSerializer.serializeContents(StdContainerSerializers.java:122)
  	at org.codehaus.jackson.map.ser.std.StdContainerSerializers$IndexedListSerializer.serializeContents(StdContainerSerializers.java:71)
  	at org.codehaus.jackson.map.ser.std.AsArraySerializerBase.serialize(AsArraySerializerBase.java:86)
  	at org.codehaus.jackson.map.ser.std.MapSerializer.serializeFields(MapSerializer.java:262)
  	at org.codehaus.jackson.map.ser.std.MapSerializer.serialize(MapSerializer.java:186)
  	at org.codehaus.jackson.map.ser.std.MapSerializer.serialize(MapSerializer.java:23)
  	at org.codehaus.jackson.map.ser.StdSerializerProvider._serializeValue(StdSerializerProvider.java:610)
  	at org.codehaus.jackson.map.ser.StdSerializerProvider.serializeValue(StdSerializerProvider.java:256)
  	at org.codehaus.jackson.map.ObjectMapper.writeValue(ObjectMapper.java:1613)
  	at org.ametys.core.util.JSONUtils.convertObjectToJson(JSONUtils.java:188)
  	... 72 more
  Caused by: java.lang.NullPointerException
  	at org.ametys.core.util.I18nUtils._translate(I18nUtils.java:291)
  	at org.ametys.core.util.I18nUtils.translate(I18nUtils.java:187)
  	at org.ametys.core.util.I18nUtils.translate(I18nUtils.java:154)
  	at org.ametys.core.util.I18nizableTextSerializer.serialize(I18nizableTextSerializer.java:70)
  	at org.ametys.core.util.I18nizableTextSerializer.serialize(I18nizableTextSerializer.java:1)
  	at org.codehaus.jackson.map.ser.std.MapSerializer.serializeFields(MapSerializer.java:262)
  	... 84 more