Fixing bug to prevent NullPointerException while doing PUT mappings

CHANGELOG.md

@@ -14,6 +14,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Enhancements
 * Introduce node level circuit breakers for k-NN [#2509](https://github.com/opensearch-project/k-NN/pull/2509)
 ### Bug Fixes
+* Fixing bug to prevent NullPointerException while doing PUT mappings [#2556](https://github.com/opensearch-project/k-NN/issues/2556)
 ### Infrastructure
 * Removed JDK 11 and 17 version from CI runs [#1921](https://github.com/opensearch-project/k-NN/pull/1921)
 * Upgrade min JDK compatibility to JDK 21 [#2422](https://github.com/opensearch-project/k-NN/pull/2422)

src/main/java/org/opensearch/knn/index/mapper/KNNVectorFieldMapper.java

@@ -161,7 +161,15 @@ public static class Builder extends ParametrizedFieldMapper.Builder {
             b.startObject(n);
             v.toXContent(b, ToXContent.EMPTY_PARAMS);
             b.endObject();
-        }), m -> m.getMethodComponentContext().getName());
+        }), m -> {
+            if (m == null) {
+                throw new IllegalArgumentException(
+                    "Mapping update for knn_vector fields is not supported. "
+                        + "Cannot update mapping without the original method configuration."
+                );
+            }
+            return m.getMethodComponentContext().getName();
+        });
 
         protected final Parameter<String> mode = Parameter.restrictedStringParam(
             KNNConstants.MODE_PARAMETER,