worldedit-core/src/main/java/com/sk89q/worldedit/LocalSession.java

@@ -100,7 +100,6 @@ import java.util.TimeZone;
 import java.util.UUID;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.atomic.AtomicBoolean;
-import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
 import java.util.stream.Collectors;
 
@@ -406,7 +405,8 @@ public class LocalSession implements TextureHolder {
      */
     public void clearHistory() {
         //FAWE start
-        if (Fawe.isMainThread() && !historyWriteLock.tryLock()) {
+        boolean mainThread = Fawe.isMainThread();
+        if (mainThread && !historyWriteLock.tryLock()) {
             // Do not make main thread wait if we cannot immediately clear history (on player logout usually)
             TaskManager.taskManager().async(this::clearHistoryTask);
             return;
@@ -414,7 +414,10 @@ public class LocalSession implements TextureHolder {
         try {
             clearHistoryTask();
         } finally {
-            historyWriteLock.unlock();
+            // only if we are on the main thread, we ever called tryLock -> need to unlock again
+            if (mainThread) {
+                historyWriteLock.unlock();
+            }
         }
     }