[SPARK-31584][WEBUI] Fix NullPointerException when parsing event log with InMemoryStore

### What changes were proposed in this pull request?
apache#27716 introduced parent index for InMemoryStore. When the method "deleteParentIndex(Object key)" in InMemoryStore.java is called and the key is not contained in "NaturalKeys v",  A java.lang.NullPointerException will be thrown. This patch fixed the issue by updating the if condition.

### Why are the changes needed?
Fixed a minor bug.

### Does this PR introduce any user-facing change?
No.

### How was this patch tested?
Added a unit test for deleteParentIndex.

Closes apache#28378 from baohe-zhang/SPARK-31584.

Authored-by: Baohe Zhang <[email protected]>
Signed-off-by: Gengliang Wang <[email protected]>

Author: Baohe Zhang

common/kvstore/src/main/java/org/apache/spark/util/kvstore/InMemoryStore.java

@@ -293,7 +293,7 @@ public boolean delete(Object key, T value) {
     private void deleteParentIndex(Object key) {
       if (hasNaturalParentIndex) {
         for (NaturalKeys v : parentToChildrenMap.values()) {
-          if (v.remove(asKey(key))) {
+          if (v.remove(asKey(key)) != null) {
             // `v` can be empty after removing the natural key and we can remove it from
             // `parentToChildrenMap`. However, `parentToChildrenMap` is a ConcurrentMap and such
             // checking and deleting can be slow.

common/kvstore/src/test/java/org/apache/spark/util/kvstore/CustomType2.java

@@ -0,0 +1,50 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.util.kvstore;
+
+public class CustomType2 {
+
+  @KVIndex(parent = "parentId")
+  public String key;
+
+  @KVIndex("id")
+  public String id;
+
+  @KVIndex("parentId")
+  public String parentId;
+
+  @Override
+  public boolean equals(Object o) {
+    if (o instanceof CustomType2) {
+      CustomType2 other = (CustomType2) o;
+      return id.equals(other.id) && parentId.equals(other.parentId);
+    }
+    return false;
+  }
+
+  @Override
+  public int hashCode() {
+    return id.hashCode() ^ parentId.hashCode();
+  }
+
+  @Override
+  public String toString() {
+    return "CustomType2[key=" + key + ",id=" + id + ",parentId=" + parentId;
+  }
+
+}

common/kvstore/src/test/java/org/apache/spark/util/kvstore/InMemoryStoreSuite.java

@@ -210,4 +210,46 @@ public void testBasicIteration() throws Exception {
     assertFalse(store.view(t1.getClass()).first(t2.id).skip(1).iterator().hasNext());
   }
 
+  @Test
+  public void testDeleteParentIndex() throws Exception {
+    KVStore store = new InMemoryStore();
+
+    CustomType2 t1 = new CustomType2();
+    t1.key = "key1";
+    t1.id = "id1";
+    t1.parentId = "parentId1";
+    store.write(t1);
+
+    CustomType2 t2 = new CustomType2();
+    t2.key = "key2";
+    t2.id = "id2";
+    t2.parentId = "parentId1";
+    store.write(t2);
+
+    CustomType2 t3 = new CustomType2();
+    t3.key = "key3";
+    t3.id = "id1";
+    t3.parentId = "parentId2";
+    store.write(t3);
+
+    CustomType2 t4 = new CustomType2();
+    t4.key = "key4";
+    t4.id = "id2";
+    t4.parentId = "parentId2";
+    store.write(t4);
+
+    assertEquals(4, store.count(CustomType2.class));
+
+    store.delete(t1.getClass(), t1.key);
+    assertEquals(3, store.count(CustomType2.class));
+
+    store.delete(t2.getClass(), t2.key);
+    assertEquals(2, store.count(CustomType2.class));
+
+    store.delete(t3.getClass(), t3.key);
+    assertEquals(1, store.count(CustomType2.class));
+
+    store.delete(t4.getClass(), t4.key);
+    assertEquals(0, store.count(CustomType2.class));
+  }
 }