Had this happen once already when editing reactor size/stabilizer size. I assume it was because I trimmed down the reactor to size 0 and build it up again somewhere else, although I can't really reproduce as of yet while building.
Another way, with the same stacktrace i believe, can easily be reproduced. Simply /destroy_entity on an entity with new reactor and stabilizer path, does the trick. Unloading the entity triggers it too
java.lang.NullPointerException at org.schema.game.client.view.effects.ConnectionDrawerManager.updateEntities(ConnectionDrawerManager.java:94) at org.schema.game.client.view.WorldDrawer.segManControllerUpdate(WorldDrawer.java:1327) at org.schema.game.client.view.WorldDrawer.update(WorldDrawer.java:677) at org.schema.game.client.view.MainGameGraphics.update(MainGameGraphics.java:689) at org.schema.schine.graphicsengine.core.GLFrame.update(GLFrame.java:848) at org.schema.schine.graphicsengine.core.GLFrame.doFrameAndUpdate(GLFrame.java:700) at org.schema.schine.graphicsengine.core.GraphicsContext.mainLoop(GraphicsContext.java:857) at org.schema.schine.graphicsengine.core.GraphicsContext.startUp(GraphicsContext.java:754) at org.schema.game.client.controller.GameMainMenuController.startGraphics(GameMainMenuController.java:1157) at org.schema.game.common.Starter.startMainMenu(Starter.java:1459) at org.schema.game.common.Starter.main(Starter.java:1003)