TINKERPOP-3140 Prevent NPE for repeat()

NPE raised during calls to hashcode() during strategy application if the repeat() child traversal was null, like if you did g.V().emit() by mistake. This change allows the existing error message we've long used to get raised. CTR

Author: spmallette

CHANGELOG.asciidoc

@@ -39,6 +39,7 @@ image::https://raw.githubusercontent.com/apache/tinkerpop/master/docs/static/ima
 * Deprecated `gremlin_python.process.__.has_key_` in favor of `gremlin_python.process.__.has_key`.
 * Added `gremlin.spark.outputRepartition` configuration to customize the partitioning of HDFS files from `OutputRDD`.
 * Allowed `mergeV()` and `mergeE()` to supply `null` in `Map` values.
+* Improved error message for when `emit()` is used without `repeat()`.
 
 [[release-3-7-3]]
 === TinkerPop 3.7.3 (October 23, 2024)

gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/branch/RepeatStep.java

@@ -179,7 +179,13 @@ public void setTraversal(final Traversal.Admin<?, ?> parentTraversal) {
 
     @Override
     public int hashCode() {
-        int result = super.hashCode() ^ this.repeatTraversal.hashCode();
+        int result = super.hashCode();
+
+        // not a normal state, but prevents NPE during strategy application allowing the better
+        // user-friendly error message to show up at iteration
+        if (this.repeatTraversal != null)
+            result ^=  repeatTraversal.hashCode();
+
         result ^= Boolean.hashCode(this.untilFirst);
         result ^= Boolean.hashCode(this.emitFirst) << 1;
         if (this.loopName != null)