Fixes an exception from being thrown on async events

src/main/java/ch/njol/skript/SkriptEventHandler.java

@@ -20,6 +20,7 @@
 
 import ch.njol.skript.lang.Trigger;
 import ch.njol.skript.timings.SkriptTimings;
+import ch.njol.skript.util.Task;
 import ch.njol.util.NonNullPair;
 import org.bukkit.Bukkit;
 import org.bukkit.event.Cancellable;
@@ -104,30 +105,30 @@ private static Iterator<Trigger> getTriggers(Class<? extends Event> event) {
 	 * @param e The Event to check.
 	 * @param priority The priority of the Event.
 	 */
-	private static void check(Event e, EventPriority priority) {
-		Iterator<Trigger> ts = getTriggers(e.getClass());
+	private static void check(Event event, EventPriority priority) {
+		Iterator<Trigger> ts = getTriggers(event.getClass());
 		if (!ts.hasNext())
 			return;
 
 		if (Skript.logVeryHigh()) {
 			boolean hasTrigger = false;
 			while (ts.hasNext()) {
 				Trigger trigger = ts.next();
-				if (trigger.getEvent().getEventPriority() == priority && trigger.getEvent().check(e)) {
+				if (trigger.getEvent().getEventPriority() == priority && trigger.getEvent().check(event)) {
 					hasTrigger = true;
 					break;
 				}
 			}
 			if (!hasTrigger)
 				return;
-			Class<? extends Event> c = e.getClass();
+			Class<? extends Event> c = event.getClass();
 			ts = getTriggers(c);
 
-			logEventStart(e);
+			logEventStart(event);
 		}
 
-		boolean isCancelled = e instanceof Cancellable && ((Cancellable) e).isCancelled() && !listenCancelled.contains(e.getClass());
-		boolean isResultDeny = !(e instanceof PlayerInteractEvent && (((PlayerInteractEvent) e).getAction() == Action.LEFT_CLICK_AIR || ((PlayerInteractEvent) e).getAction() == Action.RIGHT_CLICK_AIR) && ((PlayerInteractEvent) e).useItemInHand() != Result.DENY);
+		boolean isCancelled = event instanceof Cancellable && ((Cancellable) event).isCancelled() && !listenCancelled.contains(event.getClass());
+		boolean isResultDeny = !(event instanceof PlayerInteractEvent && (((PlayerInteractEvent) event).getAction() == Action.LEFT_CLICK_AIR || ((PlayerInteractEvent) event).getAction() == Action.RIGHT_CLICK_AIR) && ((PlayerInteractEvent) event).useItemInHand() != Result.DENY);
 
 		if (isCancelled && isResultDeny) {
 			if (Skript.logVeryHigh())
@@ -136,17 +137,21 @@ private static void check(Event e, EventPriority priority) {
 		}
 
 		while (ts.hasNext()) {
-			Trigger t = ts.next();
-			if (t.getEvent().getEventPriority() != priority || !t.getEvent().check(e))
+			Trigger trigger = ts.next();
+			if (trigger.getEvent().getEventPriority() != priority || !trigger.getEvent().check(event))
 				continue;
 
-			logTriggerStart(t);
-			Object timing = SkriptTimings.start(t.getDebugLabel());
+			logTriggerStart(trigger);
+			Object timing = SkriptTimings.start(trigger.getDebugLabel());
 
-			t.execute(e);
+			if (!Bukkit.isPrimaryThread() && !trigger.getEvent().allowAsynchronous()) {
+				Task.callSync(() -> trigger.execute(event));
+			} else {
+				trigger.execute(event);
+			}
 
 			SkriptTimings.stop(timing);
-			logTriggerEnd(t);
+			logTriggerEnd(trigger);
 		}
 
 		logEventEnd();

src/main/java/ch/njol/skript/lang/SkriptEvent.java

@@ -216,6 +216,13 @@ public boolean isEventPrioritySupported() {
 		return true;
 	}
 
+	/**
+	 * Override this method to allow Skript to not force synchronization.
+	 */
+	public boolean allowAsynchronous() {
+		return false;
+	}
+
 	/**
 	 * Fixes patterns in event by modifying every {@link ch.njol.skript.patterns.TypePatternElement}
 	 * to be nullable.

src/test/java/org/skriptlang/skript/test/tests/regression/AsyncEvents_5689.java

@@ -0,0 +1,48 @@
+/**
+ *   This file is part of Skript.
+ *
+ *  Skript is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  Skript is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with Skript.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Copyright Peter Güttinger, SkriptLang team and contributors
+ */
+package org.skriptlang.skript.test.tests.regression;
+
+import static org.easymock.EasyMock.createMock;
+
+import java.util.concurrent.CompletableFuture;
+
+import org.bukkit.Bukkit;
+import org.bukkit.entity.Player;
+import org.bukkit.event.player.AsyncPlayerChatEvent;
+import org.junit.Test;
+
+import com.google.common.collect.Sets;
+
+import ch.njol.skript.test.runner.SkriptJUnitTest;
+
+/**
+ * Issue #5689 caused an exception when attempting to do a sync required actions on an async event.
+ */
+@SuppressWarnings("deprecation") // Paper wants AsyncChatEvent for AdventureAPI
+public class AsyncEvents_5689 extends SkriptJUnitTest {
+
+	private static Player player = createMock(Player.class);
+
+	@Test
+	public void execute() {
+		AsyncPlayerChatEvent event = new AsyncPlayerChatEvent(true, player, "Issue 5689", Sets.newHashSet(player));
+		CompletableFuture.runAsync(() -> Bukkit.getPluginManager().callEvent(event));
+	}
+
+}

src/test/skript/tests/junit/AsyncEvents_5689.sk

@@ -0,0 +1,13 @@
+on script load:
+	ensure junit test "org.skriptlang.skript.test.tests.regression.AsyncEvents_5689" completes "async chat"
+
+on chat:
+	junit test is "org.skriptlang.skript.test.tests.regression.AsyncEvents_5689"
+
+	# Attempting to throw an exception
+	set {_block} to type of block at spawn of "world"
+	set block at spawn of "world" to stone
+	set block at spawn of "world" to {_block}
+	execute console command "command doesn't exist"
+	wait a tick
+	complete objective "async chat" for junit test "org.skriptlang.skript.test.tests.regression.AsyncEvents_5689"