Fixes an exception from being thrown on async events

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

@@ -18,11 +18,17 @@
  */
 package ch.njol.skript;
 
-import ch.njol.skript.lang.SkriptEvent;
-import ch.njol.skript.lang.Trigger;
-import ch.njol.skript.timings.SkriptTimings;
-import com.google.common.collect.ArrayListMultimap;
-import com.google.common.collect.Multimap;
+import java.lang.ref.WeakReference;
+import java.lang.reflect.Method;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+import java.util.stream.Collectors;
+
 import org.bukkit.Bukkit;
 import org.bukkit.event.Cancellable;
 import org.bukkit.event.Event;
@@ -36,16 +42,13 @@
 import org.bukkit.plugin.RegisteredListener;
 import org.eclipse.jdt.annotation.Nullable;
 
-import java.lang.ref.WeakReference;
-import java.lang.reflect.Method;
-import java.util.HashMap;
-import java.util.HashSet;
-import java.util.Iterator;
-import java.util.List;
-import java.util.Map;
-import java.util.Map.Entry;
-import java.util.Set;
-import java.util.stream.Collectors;
+import com.google.common.collect.ArrayListMultimap;
+import com.google.common.collect.Multimap;
+
+import ch.njol.skript.lang.SkriptEvent;
+import ch.njol.skript.lang.Trigger;
+import ch.njol.skript.timings.SkriptTimings;
+import ch.njol.skript.util.Task;
 
 public final class SkriptEventHandler {
 
@@ -110,12 +113,14 @@ private static List<Trigger> getTriggers(Class<? extends Event> event) {
 	 */
 	private static void check(Event event, EventPriority priority) {
 		List<Trigger> triggers = getTriggers(event.getClass());
+		if (triggers.isEmpty())
+			return;
 
 		if (Skript.logVeryHigh()) {
 			boolean hasTrigger = false;
 			for (Trigger trigger : triggers) {
 				SkriptEvent triggerEvent = trigger.getEvent();
-				if (triggerEvent.getEventPriority() == priority && triggerEvent.check(event)) {
+				if (triggerEvent.getEventPriority() == priority && Boolean.TRUE.equals(Task.callSync(() -> triggerEvent.check(event)))) {
 					hasTrigger = true;
 					break;
 				}
@@ -137,16 +142,31 @@ private static void check(Event event, EventPriority priority) {
 
 		for (Trigger trigger : triggers) {
 			SkriptEvent triggerEvent = trigger.getEvent();
-			if (triggerEvent.getEventPriority() != priority || !triggerEvent.check(event))
+			if (triggerEvent.getEventPriority() != priority)
 				continue;
 
-			logTriggerStart(trigger);
-			Object timing = SkriptTimings.start(trigger.getDebugLabel());
-
-			trigger.execute(event);
-
-			SkriptTimings.stop(timing);
-			logTriggerEnd(trigger);
+			// these methods need to be run on whatever thread the trigger is
+			Runnable execute = () -> {
+				logTriggerStart(trigger);
+				Object timing = SkriptTimings.start(trigger.getDebugLabel());
+				trigger.execute(event);
+				SkriptTimings.stop(timing);
+				logTriggerEnd(trigger);
+			};
+
+			if (trigger.getEvent().canExecuteAsynchronously()) {
+				// check should be performed on the main thread
+				if (Boolean.FALSE.equals(Task.callSync(() -> triggerEvent.check(event))))
+					continue;
+				execute.run();
+			} else { // Ensure main thread
+				Task.callSync(() -> {
+					if (!triggerEvent.check(event))
+						return null;
+					execute.run();
+					return null; // we don't care about a return value
+				});
+			}
 		}
 
 		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 canExecuteAsynchronously() {
+		return false;
+	}
+
 	/**
 	 * Fixes patterns in event by modifying every {@link ch.njol.skript.patterns.TypePatternElement}
 	 * to be nullable.