Guard against JS error by rendering arrows only if needed

Author: Andrew Or

core/src/main/resources/org/apache/spark/ui/static/spark-dag-viz.js

@@ -334,11 +334,13 @@ function drawCrossStageEdges(edges, svgContainer) {
   }
   // Now draw the arrows by borrowing the arrow marker generated by dagre-d3
   var dagreD3Marker = svgContainer.select("g.edgePaths marker").node();
-  svgContainer
-    .append(function() { return dagreD3Marker.cloneNode(true); })
-    .attr("id", "marker-arrow")
-  svgContainer.selectAll("g > path").attr("marker-end", "url(#marker-arrow)");
-  svgContainer.selectAll("g.edgePaths def").remove(); // We no longer need these
+  if (!dagreD3Marker.empty()) {
+    svgContainer
+      .append(function() { return dagreD3Marker.cloneNode(true); })
+      .attr("id", "marker-arrow")
+    svgContainer.selectAll("g > path").attr("marker-end", "url(#marker-arrow)");
+    svgContainer.selectAll("g.edgePaths def").remove(); // We no longer need these
+  }
 }
 
 /*