Support null exception in ErrorMessage (opensearch-project#2900) (opensearch-project#2920)

* null exception

Signed-off-by: Jing Zhang <jngz@amazon.com>

* address comments

Signed-off-by: Jing Zhang <jngz@amazon.com>

---------

Signed-off-by: Jing Zhang <jngz@amazon.com>
(cherry picked from commit 55d28e0)

Co-authored-by: Jing Zhang <jngz@amazon.com>

Author: opensearch-trigger-bot[bot]

plugin/src/main/java/org/opensearch/ml/utils/error/ErrorMessage.java

@@ -44,14 +44,18 @@ public ErrorMessage(Throwable exception, int status) {
     }
 
     private String fetchType() {
-        return exception.getClass().getSimpleName();
+        return exception == null ? "Unknown Exception" : exception.getClass().getSimpleName();
     }
 
     protected String fetchReason() {
         return status == RestStatus.BAD_REQUEST.getStatus() ? "Invalid Request" : "System Error";
     }
 
     protected String fetchDetails() {
+        if (exception == null) {
+            return "No Exception Details";
+        }
+
         final String msg;
         // Prevent the method from exposing internal information such as internal ip address etc. that is a security concern.
         if (hasInternalInformation(exception)) {

plugin/src/test/java/org/opensearch/ml/utils/error/ErrorMessageTests.java

@@ -144,4 +144,12 @@ public void getDetails() {
 
         assertEquals(errorMessage.getDetails(), "illegal state");
     }
+
+    @Test
+    public void ConstructNullException() {
+        ErrorMessage errorMessage = new ErrorMessage(null, SERVICE_UNAVAILABLE.getStatus());
+
+        assertEquals(errorMessage.getType(), "Unknown Exception");
+        assertEquals(errorMessage.getDetails(), "No Exception Details");
+    }
 }