Steps to reproduce:

1. start a Camunda Wildfly distribution
2. build the attached project and deploy it on wildfly
3. start a new instance in tasklist (by using the engine "foo")
4. complete task "A"
5. try to open external form of task "B"

Observed Behavior:

• The variable cannot be serialized, because the Process-application-specific ObjectMapper is not used to serialize the variable
• The following exception is thrown:

  Caused by: org.camunda.bpm.engine.ProcessEngineException: Cannot serialize object in variable 'null': SPIN/JACKSON-JSON-01009 Unable to map object '[ModellingEntity[id=7]]' to json node
  	at org.camunda.bpm.engine.impl.variable.serializer.AbstractSerializableValueSerializer.writeValue(AbstractSerializableValueSerializer.java:53)
  	at org.camunda.bpm.engine.impl.variable.serializer.AbstractSerializableValueSerializer.writeValue(AbstractSerializableValueSerializer.java:27)
  	at org.camunda.bpm.engine.impl.persistence.entity.util.TypedValueField.writeValue(TypedValueField.java:160)
  	at org.camunda.bpm.engine.impl.persistence.entity.util.TypedValueField.isValuedImplicitlyUpdated(TypedValueField.java:148)
  	at org.camunda.bpm.engine.impl.persistence.entity.util.TypedValueField.notifyImplicitValueUpdate(TypedValueField.java:169)
  	at org.camunda.bpm.engine.impl.persistence.entity.util.TypedValueField.onCommandContextClose(TypedValueField.java:165)
  	at org.camunda.bpm.engine.impl.interceptor.CommandContext.fireCommandContextClose(CommandContext.java:186)
  	at org.camunda.bpm.engine.impl.interceptor.CommandContext.close(CommandContext.java:131)
  	at org.camunda.bpm.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:113)
  	at org.camunda.bpm.engine.impl.interceptor.JtaTransactionInterceptor.execute(JtaTransactionInterceptor.java:58)
  	at org.camunda.bpm.engine.impl.interceptor.ProcessApplicationContextInterceptor.execute(ProcessApplicationContextInterceptor.java:66)
  	at org.camunda.bpm.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:30)
  	at org.camunda.bpm.engine.impl.RuntimeServiceImpl.getVariablesTyped(RuntimeServiceImpl.java:295)
  	at org.camunda.bpm.engine.impl.RuntimeServiceImpl.getVariables(RuntimeServiceImpl.java:290)
  	at org.camunda.bpm.engine.impl.RuntimeServiceImpl.getVariables(RuntimeServiceImpl.java:75)
  	at com.hannoverre.lsr.faces.model.UserTaskForm.initUserTask(UserTaskForm.java:113)
  	... 89 more
  Caused by: org.camunda.spin.json.SpinJsonDataFormatException: SPIN/JACKSON-JSON-01009 Unable to map object '[ModellingEntity[id=7]]' to json node
  	at org.camunda.spin.impl.json.jackson.JacksonJsonLogger.unableToMapInput(JacksonJsonLogger.java:80)
  	at org.camunda.spin.impl.json.jackson.format.JacksonJsonDataFormatMapper.mapJavaToInternal(JacksonJsonDataFormatMapper.java:54)
  	at org.camunda.spin.plugin.impl.SpinObjectValueSerializer.serializeToByteArray(SpinObjectValueSerializer.java:72)
  	at org.camunda.bpm.engine.impl.variable.serializer.AbstractSerializableValueSerializer.writeValue(AbstractSerializableValueSerializer.java:50)
  	... 104 more
  Caused by: java.lang.IllegalArgumentException: Can not resolve PropertyFilter with id 'ModellingEntity'; no FilterProvider configured (through reference chain: java.util.ArrayList[0])
  	at com.fasterxml.jackson.databind.ObjectMapper.valueToTree(ObjectMapper.java:2520)
  	at org.camunda.spin.impl.json.jackson.format.JacksonJsonDataFormatMapper.mapJavaToInternal(JacksonJsonDataFormatMapper.java:52)
  	... 106 more
  Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not resolve PropertyFilter with id 'ModellingEntity'; no FilterProvider configured (through reference chain: java.util.ArrayList[0])
  	at com.fasterxml.jackson.databind.ser.std.StdSerializer.findPropertyFilter(StdSerializer.java:294)
  	at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFieldsFiltered(BeanSerializerBase.java:714)
  	at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
  	at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:119)
  	at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serialize(IndexedListSerializer.java:79)
  	at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serialize(IndexedListSerializer.java:18)
  	at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:130)
  	at com.fasterxml.jackson.databind.ObjectMapper.writeValue(ObjectMapper.java:2383)
  	at com.fasterxml.jackson.databind.ObjectMapper.valueToTree(ObjectMapper.java:2515)
  	... 107 more
  

Expected Behavior:

• The variable can be serialized by using the Process-application-specific ObjectMapper