AdvancedFileOutput: Fix race condition while flushing the buffer, fixes #99

Author: Bungeefan

lib/src/outputs/advanced_file_output.dart

@@ -217,13 +217,15 @@ class AdvancedFileOutput extends LogOutput {
   }
 
   Future<void> _closeSink() async {
+    final sink = _sink;
+    _sink = null; // disable writing in flushBuffer
+
     if (fileFooter != null) {
       _sink?.writeln(fileFooter);
     }
 
-    await _sink?.flush();
-    await _sink?.close();
-    _sink = null; // Explicitly set null until assigned again
+    await sink?.flush();
+    await sink?.close();
   }
 
   Future<void> _deleteRotatedFiles() async {