If your fill tool is set on a build helper grid (filling empty air) and is restricted, fill a few times, then you recalculate the set build helper to be outside, and use the fill tool again. You may experience the crash below, seems to be inconsistent/rare.
java.lang.NullPointerException at it.unimi.dsi.fastutil.longs.LongOpenHashSet$SetIterator.nextLong(LongOpenHashSet.java:343) at org.schema.game.client.controller.manager.ingame.FillTool.doFill(FillTool.java:83) at org.schema.game.client.view.gui.advancedbuildmode.AdvancedBuildModeFill$2$2.pressedLeftMouse(AdvancedBuildModeFill.java:150) at org.schema.game.client.view.gui.advanced.tools.ButtonResult.leftClick(ButtonResult.java:30) at org.schema.game.client.view.gui.advanced.tools.GUIAdvButton$1.callback(GUIAdvButton.java:28) at org.schema.schine.network.client.GUICallbackController.execute(GUICallbackController.java:64) at org.schema.schine.input.BasicInputController.updateInput(BasicInputController.java:234) at org.schema.game.client.controller.GameClientController.updateStateInput(GameClientController.java:1461) at org.schema.schine.graphicsengine.core.GLFrame.doFrameAndUpdate(GLFrame.java:667) at org.schema.schine.graphicsengine.core.GraphicsContext.mainLoop(GraphicsContext.java:808) at org.schema.schine.graphicsengine.core.GraphicsContext.startUp(GraphicsContext.java:705) at org.schema.game.client.controller.GameMainMenuController.startGraphics(GameMainMenuController.java:1142) at org.schema.game.common.Starter.startMainMenu(Starter.java:1450) at org.schema.game.common.Starter.main(Starter.java:995)