Avoid errors

src/main/java/it/cavallium/dbengine/database/disk/LLLocalLuceneIndex.java

@@ -280,7 +280,9 @@ public class LLLocalLuceneIndex implements LLLuceneIndex {
 
 		@Override
 		public void run() {
-			scheduledTasksLifecycle.startScheduledTask();
+			if (!scheduledTasksLifecycle.tryStartScheduledTask()) {
+				return;
+			}
 			try {
 				if (scheduledTasksLifecycle.isCancelled() || cancelled) return;
 				task.run();

src/main/java/it/cavallium/dbengine/lucene/ScheduledTaskLifecycle.java

@@ -27,6 +27,19 @@ public class ScheduledTaskLifecycle {
 		this.lock.readLock();
 	}
 
+	/**
+	 * Mark this task as running.
+	 * After calling this method, please call {@method endScheduledTask} inside a finally block!
+	 * @return false if failed
+	 */
+	public boolean tryStartScheduledTask() {
+		if (cancelled) {
+			return false;
+		}
+		this.lock.readLock();
+		return true;
+	}
+
 	/**
 	 * Mark this task as ended. Must be called after {@method startScheduledTask}
 	 */