Mining model is not able to chain regression models

Mining model is not able to chain regression models. See reproducer in PR: https://github.com/kiegroup/drools/pull/2057

It fails with following exception:
java.lang.NullPointerException: null
at org.kie.pmml.pmml_4_2.model.AbstractModel.getOutputFields(AbstractModel.java:385)
at org.kie.pmml.pmml_4_2.model.Miningmodel.getChildOutputField(Miningmodel.java:87)
at org.kie.pmml.pmml_4_2.model.Miningmodel.findOutputField(Miningmodel.java:113)
at org.kie.pmml.pmml_4_2.model.AbstractModel.getMiningFields(AbstractModel.java:338)
at org.kie.pmml.pmml_4_2.model.Miningmodel.lambda$getMiningFields$2(Miningmodel.java:127)