Protect against the leader index being removed (#39351)

when dealing with TimeoutException

The `IndexFollowingIT#testDeleteLeaderIndex()`` test failed,
because a NPE was captured as fatal error instead of an IndexNotFoundException.

Closes #39308

Author: martijnvg

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/ShardChangesAction.java

@@ -24,6 +24,7 @@
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.unit.ByteSizeValue;
 import org.elasticsearch.common.unit.TimeValue;
+import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.index.IndexService;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.engine.MissingHistoryOperationsException;
@@ -418,6 +419,11 @@ private void globalCheckpointAdvancementFailure(
             if (e instanceof TimeoutException) {
                 try {
                     final IndexMetaData indexMetaData = clusterService.state().metaData().index(shardId.getIndex());
+                    if (indexMetaData == null) {
+                        listener.onFailure(new IndexNotFoundException(shardId.getIndex()));
+                        return;
+                    }
+
                     final long mappingVersion = indexMetaData.getMappingVersion();
                     final long settingsVersion = indexMetaData.getSettingsVersion();
                     final SeqNoStats latestSeqNoStats = indexShard.seqNoStats();

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/ccr/IndexFollowingIT.java

@@ -660,7 +660,6 @@ public void testCloseFollowIndex() throws Exception {
         pauseFollow("index2");
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/39308")
     public void testDeleteLeaderIndex() throws Exception {
         assertAcked(leaderClient().admin().indices().prepareCreate("index1")
             .setSettings(Settings.builder()