Error message of empty <secondaryEntitySelector> with multiple entity classes must be clearer

Exception in thread "main" java.lang.IllegalArgumentException: The entitySelector (EntitySelectorConfig(class org.optaplanner.examples.examination.domain.LeadingExam))'s entityDescriptor (EntityDescriptor(org.optaplanner.examples.examination.domain.LeadingExam)) and secondaryEntitySelectorConfig (EntitySelectorConfig(null))'s entityDescriptor (null) must have the same entity class.
	at org.optaplanner.core.config.heuristic.selector.move.generic.SwapMoveSelectorConfig.buildUnfoldedMoveSelectorConfig(SwapMoveSelectorConfig.java:109)
	at org.optaplanner.core.config.heuristic.selector.move.MoveSelectorConfig.buildMoveSelector(MoveSelectorConfig.java:181)
	at org.optaplanner.core.config.heuristic.selector.move.composite.UnionMoveSelectorConfig.buildBaseMoveSelector(UnionMoveSelectorConfig.java:77)
	at org.optaplanner.core.config.heuristic.selector.move.MoveSelectorConfig.buildMoveSelector(MoveSelectorConfig.java:194)
	at org.optaplanner.core.config.localsearch.LocalSearchPhaseConfig.buildMoveSelector(LocalSearchPhaseConfig.java:237)
	at org.optaplanner.core.config.localsearch.LocalSearchPhaseConfig.buildDecider(LocalSearchPhaseConfig.java:128)
	at org.optaplanner.core.config.localsearch.LocalSearchPhaseConfig.buildPhase(LocalSearchPhaseConfig.java:114)
	at org.optaplanner.core.config.localsearch.LocalSearchPhaseConfig.buildPhase(LocalSearchPhaseConfig.java:51)
	at org.optaplanner.core.config.solver.SolverConfig.buildPhaseList(SolverConfig.java:297)
	at org.optaplanner.core.config.solver.SolverConfig.buildSolver(SolverConfig.java:233)
	at org.optaplanner.core.impl.solver.AbstractSolverFactory.buildSolver(AbstractSolverFactory.java:61)
	at org.optaplanner.examples.common.app.CommonApp.createSolver(CommonApp.java:138)
	at org.optaplanner.examples.common.app.CommonApp.createSolutionBusiness(CommonApp.java:127)
	at org.optaplanner.examples.common.app.CommonApp.init(CommonApp.java:118)
	at org.optaplanner.examples.common.app.CommonApp.init(CommonApp.java:114)
	at org.optaplanner.examples.examination.app.ExaminationApp.main(ExaminationApp.java:43)