Showing decision tree for the dating problem throws exception

[Manuel Luque]

Repro steps: - Open dating network (attached) - Hit the "Decision tree" button Nothing happens (the decision tree is not shown) and there's the stack trace of this exception in the Message Window with the next first messages: Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at org.openmarkov.core.dt.DecisionTreeNode.<init>(DecisionTreeNode.java:35) at org.openmarkov.core.dt.DecisionTreeBuilder.addUtilityNodes(DecisionTreeBuilder.java:365) ...