Planning list variable is throwing an error. How to solve it?

[aayuwush]

Exception in thread "main" java.lang.IllegalStateException: The planning list variable (Machine.tasks variable) of entity (com.ai.solver.timefold01.domains.Machine@26bbe604) is null.
at ai.timefold.solver.core.impl.domain.variable.descriptor.ListVariableDescriptor.getValue(ListVariableDescriptor.java:153)
at ai.timefold.solver.core.impl.domain.variable.descriptor.ListVariableDescriptor.getListSize(ListVariableDescriptor.java:181)
at ai.timefold.solver.core.impl.domain.solution.descriptor.SolutionDescriptor.lambda$computeInitializationStatistics$16(SolutionDescriptor.java:1165)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
at ai.timefold.solver.core.impl.domain.solution.descriptor.SolutionDescriptor.lambda$visitAllEntities$8(SolutionDescriptor.java:909)
at ai.timefold.solver.core.impl.domain.solution.descriptor.SolutionDescriptor.visitAllEntities(SolutionDescriptor.java:923)
at ai.timefold.solver.core.impl.domain.solution.descriptor.SolutionDescriptor.visitAllEntities(SolutionDescriptor.java:909)
at ai.timefold.solver.core.impl.domain.solution.descriptor.SolutionDescriptor.computeInitializationStatistics(SolutionDescriptor.java:1141)
at ai.timefold.solver.core.impl.score.director.AbstractScoreDirector.setWorkingSolution(AbstractScoreDirector.java:225)
at ai.timefold.solver.constraint.streams.bavet.BavetConstraintStreamScoreDirector.setWorkingSolution(BavetConstraintStreamScoreDirector.java:42)
at ai.timefold.solver.core.impl.solver.scope.SolverScope.setWorkingSolutionFromBestSolution(SolverScope.java:252)
at ai.timefold.solver.core.impl.solver.AbstractSolver.solvingStarted(AbstractSolver.java:64)
at ai.timefold.solver.core.impl.solver.DefaultSolver.solvingStarted(DefaultSolver.java:228)
at ai.timefold.solver.core.impl.solver.DefaultSolver.solve(DefaultSolver.java:199)
at com.ai.solver.timefold01.TaskScheduleApplication.main(TaskScheduleApplication.java:32)

[triceo]

Hello @aayuwush.

The error message is telling you what's wrong - the tasks variable on Machine entity is null. Make it not null by setting an empty ArrayList to it.