Hardening of some sort needed in DelayedActionSystem (plant growth can cause NPE crashes?)

[Cervator]

During multiplayer event the server crashed a minute after @rzats had himself crashed out via Terasology/JoshariasSurvival#14 shortly after I crashed out via #2361

I suspect something about the blueberry bush growth stages from JoshariasSurvival, which is based off SimpleFarming which in turn uses the DelayManager system from the engine. If I were to wager a guess there was an immature blueberry bush in a chunk one of us had been in that had its growth tick in a way that was no longer valid with no players left on the server.

Crash happened in DelayedActionSystem and while the log doesn't identify exactly what the scheduled action was I think a fair bet would be a blueberry bush. That in turn is defined in BlueberrySeed.prefab from JoshariasSurvival, which has a PlantDefinitionComponent that still has a bit of logic and other fanciness despite being a Component (old code I suspect). The related logic system is FarmingAuthoritySystem which is what in turn uses the DelayManager.

So some things to consider looking at:

• Find out what causes the (likely) blueberry growth to result in an invalid scheduled action (something goes stale as player(s) disappear?)
• Harden DelayedActionSystem to where it has error handling for encountering invalid scheduled actions (expect that invalid tasks could be created somehow)
• Log some better details when a bad action is encountered (which one is it? Where did it come from?)
• Look at SimpleFarming's use of delayed actions (probably predates the refactoring @xrtariq2594 applied to the delay system and wouldn't have been an obvious thing to test)
• Simplify PlantDefinitionComponent also from SimpleFarming to follow code guidelines (no logic in Components, keep them basic)
• Bonus: Add some more documentation to the scheduled task system, maybe some basic examples or even a little tutorial somewhere?
• Edit addition: It seems fairly reproducible in multiplayer that breaking a blueberry bush (after picking the berries?) can cause a crash. Maybe not directly related, could be more about Multiplayer crash affecting multiple clients (but not server?) #2361

Error log below (server side):

14:01:59.002 [main] WARN  o.t.l.c.ServerCharacterPredictionSystem - Received too much input from EntityRef{id = 9050, netId = 20052, prefab = 'engine:player'}, dropping input.
14:01:59.007 [main] WARN  o.t.l.c.ServerCharacterPredictionSystem - Received too much input from EntityRef{id = 9050, netId = 20052, prefab = 'engine:player'}, dropping input.
14:01:59.013 [main] WARN  o.t.l.c.ServerCharacterPredictionSystem - Received too much input from EntityRef{id = 9050, netId = 20052, prefab = 'engine:player'}, dropping input.
14:02:07.843 [main] INFO  o.t.n.internal.NetworkSystemImpl - Client disconnected: rzats
14:02:08.237 [main] INFO  o.t.n.internal.NetworkSystemImpl - Client disconnected: rzats
14:02:08.726 [main] INFO  o.t.p.i.ReadWriteStorageManager - Saving - Creating game snapshot
14:02:08.727 [main] INFO  o.t.p.i.ReadWriteStorageManager - Saving - Snapshot created: Writing phase starts
14:02:11.916 [main] INFO  o.t.p.i.ReadWriteStorageManager - Saving - Creating game snapshot
14:02:11.916 [main] INFO  o.t.p.i.ReadWriteStorageManager - Saving - Snapshot created: Writing phase starts
14:03:05.293 [main] ERROR o.terasology.engine.TerasologyEngine - Uncaught exception, attempting clean game shutdown
java.lang.NullPointerException: null
    at org.terasology.logic.delay.DelayedActionSystem.lambda$invokeDelayedOperations$271(DelayedActionSystem.java:78)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
    at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
    at java.util.LinkedList$LLSpliterator.forEachRemaining(LinkedList.java:1235)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
    at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
    at org.terasology.logic.delay.DelayedActionSystem.invokeDelayedOperations(DelayedActionSystem.java:75)
    at org.terasology.logic.delay.DelayedActionSystem.update(DelayedActionSystem.java:58)
    at org.terasology.engine.modes.StateIngame.update(StateIngame.java:176)
    at org.terasology.engine.TerasologyEngine.mainLoop(TerasologyEngine.java:413)
    at org.terasology.engine.TerasologyEngine.run(TerasologyEngine.java:368)
    at org.terasology.engine.Terasology.main(Terasology.java:141)
14:03:05.294 [main] INFO  o.terasology.engine.TerasologyEngine - Shutting down Terasology...
14:03:05.306 [main] INFO  o.t.p.i.ReadWriteStorageManager - Saving - Creating game snapshot
14:03:05.307 [main] INFO  o.t.p.i.ReadWriteStorageManager - Saving - Snapshot created: Writing phase starts
14:03:05.322 [main] INFO  o.t.n.internal.NetworkSystemImpl - Network shutdown

[xrtariq2594]

Item 5 has been improved in the develop branch of SimpleFarming.