Manage null CallSite receivers in script-inset

runtime/observatory/lib/src/elements/script_inset.dart

Index: runtime/observatory/lib/src/elements/script_inset.dart
diff --git a/runtime/observatory/lib/src/elements/script_inset.dart b/runtime/observatory/lib/src/elements/script_inset.dart
index 18ac0db6dc4099bcd5160170028c66fbec4c4174..7413051488dc215ff90f91ed704124f2bdabdcb4 100644
--- a/runtime/observatory/lib/src/elements/script_inset.dart
+++ b/runtime/observatory/lib/src/elements/script_inset.dart
@@ -1181,7 +1181,11 @@ class CallSiteAnnotation extends Annotation {
 
         for (var entry in callSite.entries) {
           var r = row();
-          r.append(cell(serviceRef(entry.receiver)));
+          if (entry.receiver == null) {
+            r.append(cell(""));
+          } else {
+            r.append(cell(serviceRef(entry.receiver)));
+          }
           r.append(cell(entry.count.toString()));
           r.append(cell(serviceRef(entry.target)));
           details.append(r);