Simple test case:

$ cat FreeBuffer.java
import sun.nio.ch.Util;
import java.nio.ByteBuffer;

public class FreeBuffer {

    public static void main(String[] args) {
        // Get a buffer with a capacity of 100, and then get its slice object.
        // The slice cleaner is empty.
        // And there is no reference to the allocated buffer.
        ByteBuffer slice = ByteBuffer.allocateDirect(100).slice();

        // Release this slice and it will be in the cache.
        Util.releaseTemporaryDirectBuffer(slice);

        // Get a buffer with a capacity of 1000.
        // At this time, the slice in the cache will be freed by Util.free().
        // But the cleaner of this slice is empty.
        Util.getTemporaryDirectBuffer(1000);
    }

}

$ javac --add-exports=java.base/sun.nio.ch=ALL-UNNAMED FreeBuffer.java

$ java FreeBuffer
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "jdk.internal.ref.Cleaner.clean()" because the return value of "sun.nio.ch.DirectBuffer.cleaner()" is null
        at java.base/sun.nio.ch.Util.free(Util.java:328)
        at java.base/sun.nio.ch.Util.getTemporaryDirectBuffer(Util.java:241)
        at FreeBuffer.main(FreeBuffer.java:18)

Proposed fix:
diff -r a37f2e2b6fee src/java.base/share/classes/sun/nio/ch/Util.java
--- a/src/java.base/share/classes/sun/nio/ch/Util.java Mon Jul 06 23:11:37 2020 -0700
+++ b/src/java.base/share/classes/sun/nio/ch/Util.java Tue Jul 07 15:20:18 2020 +0800
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -40,6 +40,7 @@
 import jdk.internal.access.foreign.MemorySegmentProxy;
 import jdk.internal.misc.TerminatingThreadLocal;
 import jdk.internal.misc.Unsafe;
+import jdk.internal.ref.Cleaner;
 import sun.security.action.GetPropertyAction;

 public class Util {
@@ -322,10 +323,14 @@
     }

     /**
- * Frees the memory for the given direct buffer
+ * Frees the memory for the given direct buffer if the buffer has cleaner.
+ * When the buffer has no cleaner, it will be recycled at GC time.
      */
     private static void free(ByteBuffer buf) {
- ((DirectBuffer)buf).cleaner().clean();
+ Cleaner cleaner = ((DirectBuffer)buf).cleaner();
+ if (cleaner != null) {
+ cleaner.clean();
+ }
     }