Add undefined check to _computeParents

Change-Id: I74f9769ace6b53dc4a634c93ad9736e21b05f474
diff --git a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.js b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.js
index 010c304..2b6528a 100644
--- a/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.js
+++ b/polygerrit-ui/app/elements/change/gr-change-metadata/gr-change-metadata.js
@@ -438,7 +438,7 @@
     },
 
     _computeParents(change) {
-      if (!change.current_revision ||
+      if (!change || !change.current_revision ||
           !change.revisions[change.current_revision] ||
           !change.revisions[change.current_revision].commit) {
         return undefined;