I haven't seen this in vanilla, and I do have a lot of mods installed, but the stack trace is pure vanilla... move it to the modded bugs section if you need to.
6648934 [Thread-5] ERROR com.fs.starfarer.combat.CombatMain - java.lang.NullPointerException
java.lang.NullPointerException
at com.fs.starfarer.api.impl.campaign.events.nearby.NearbyEventsEvent.generateDistressCallAmbushTrap(NearbyEventsEvent.java:307)
at com.fs.starfarer.api.impl.campaign.events.nearby.NearbyEventsEvent.maybeSpawnDistressCall(NearbyEventsEvent.java:258)
at com.fs.starfarer.api.impl.campaign.events.nearby.NearbyEventsEvent.advance(NearbyEventsEvent.java:136)
at com.fs.starfarer.campaign.events.CampaignEventManager.advance(Unknown Source)
at com.fs.starfarer.campaign.CampaignEngine.advance(Unknown Source)
at com.fs.starfarer.campaign.CampaignState.advance(Unknown Source)
at com.fs.starfarer.BaseGameState.traverse(Unknown Source)
at com.fs.state.AppDriver.begin(Unknown Source)
at com.fs.starfarer.combat.CombatMain.main(Unknown Source)
at com.fs.starfarer.StarfarerLauncher$1.run(Unknown Source)
at java.lang.Thread.run(Thread.java:745)
Possibly relevant information: All three of the nearest systems were occupied when this happened; one contained my colony, and the other two contain pirate stations.
Edit: Correction: The single nearest system was unoccupied - but I've noticed that distress calls never spawn in the literal closest system to where your fleet is. The three next-closest all were occupied as described above, and there were no other systems in the constellation or otherwise nearby.