Fix the potential multi-concurrency issues that may arise from high-concurrency…

Flutter/ios/Classes/FTXPlayerEventSinkQueue.m

@@ -48,6 +48,7 @@
 
 - (void)flushIfNeed
 {
+    @synchronized(self) {
         if (self.eventSink == nil) {
             return;
         }
@@ -58,11 +59,14 @@
             self.eventSink(obj);
         }
         [self.eventQueue removeAllObjects];
+    }
 }
 
 - (void)enqueue:(NSObject *)event
 {
+    @synchronized (self) {
         [self.eventQueue addObject:event];
+    }
 }
 
 @end