add double guard for closing the writer.

Author: jeme

src/DotJEM.Json.Index2/IO/JsonIndexWriterManager.cs

@@ -48,6 +48,9 @@ public void Close()
 
         lock (padlock)
         {
+            if (!writer.IsValueCreated)
+                return;
+
             writer.Value.Dispose();
             writer = new(() => Open(index));
             RaiseOnClose();