NPE in Cheap Time Scheduling with "group by" related to comparator (regression)

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at java.util.Comparator.lambda$comparingLong$6043328a$1(Comparator.java:511)
	at java.util.Comparator.lambda$comparing$ea9a8b3a$1(Comparator.java:436)
	at java.util.Comparator.lambda$thenComparing$36697e65$1(Comparator.java:216)
	at java.util.Comparator.lambda$thenComparing$36697e65$1(Comparator.java:216)
	at java.util.Comparator.lambda$thenComparing$36697e65$1(Comparator.java:216)
	at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
	at java.util.TimSort.sort(TimSort.java:234)
	at java.util.Arrays.sort(Arrays.java:1512)
	at java.util.ArrayList.sort(ArrayList.java:1462)
	at java.util.Collections.sort(Collections.java:175)
	at org.optaplanner.examples.cheaptime.swingui.CheapTimePanel.createTaskAssignmentPlot(CheapTimePanel.java:132)
	at org.optaplanner.examples.cheaptime.swingui.CheapTimePanel.createChart(CheapTimePanel.java:97)
	at org.optaplanner.examples.cheaptime.swingui.CheapTimePanel.resetPanel(CheapTimePanel.java:89)
	at org.optaplanner.examples.cheaptime.swingui.CheapTimePanel.resetPanel(CheapTimePanel.java:59)
	at org.optaplanner.examples.common.swingui.SolverAndPersistenceFrame.resetScreen(SolverAndPersistenceFrame.java:724)
	at org.optaplanner.examples.common.swingui.SolverAndPersistenceFrame.setSolutionLoaded(SolverAndPersistenceFrame.java:690)
	at org.optaplanner.examples.common.swingui.SolverAndPersistenceFrame$QuickOpenAction.actionPerformed(SolverAndPersistenceFrame.java:250)
	at org.optaplanner.examples.common.swingui.SolverAndPersistenceFrame.lambda$createQuickOpenPanel$0(SolverAndPersistenceFrame.java:205)
	at javax.swing.JList.fireSelectionValueChanged(JList.java:1802)
	at javax.swing.JList$ListSelectionHandler.valueChanged(JList.java:1816)
	at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:184)
	at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:154)
	at javax.swing.DefaultListSelectionModel.setValueIsAdjusting(DefaultListSelectionModel.java:685)
	at javax.swing.JList.setValueIsAdjusting(JList.java:2146)
	at javax.swing.plaf.basic.BasicListUI$Handler.mouseReleased(BasicListUI.java:2796)
	at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:290)
	at java.awt.Component.processMouseEvent(Component.java:6539)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
	at java.awt.Component.processEvent(Component.java:6304)
	at java.awt.Container.processEvent(Container.java:2239)
	at java.awt.Component.dispatchEventImpl(Component.java:4889)
	at java.awt.Container.dispatchEventImpl(Container.java:2297)
	at java.awt.Component.dispatchEvent(Component.java:4711)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4904)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4535)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4476)
	at java.awt.Container.dispatchEventImpl(Container.java:2283)
	at java.awt.Window.dispatchEventImpl(Window.java:2746)
	at java.awt.Component.dispatchEvent(Component.java:4711)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:760)
	at java.awt.EventQueue.access$500(EventQueue.java:97)
	at java.awt.EventQueue$3.run(EventQueue.java:709)
	at java.awt.EventQueue$3.run(EventQueue.java:703)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:84)
	at java.awt.EventQueue$4.run(EventQueue.java:733)
	at java.awt.EventQueue$4.run(EventQueue.java:731)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:74)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:730)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:205)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)