Add undefined check to _handleRevisionUpdate

Change-Id: Ib3be7451108f0ba72ffe076c28caceb75e1fc6bc
diff --git a/gr-checks/gr-checks-view.js b/gr-checks/gr-checks-view.js
index f27eb11..5f4a11f 100644
--- a/gr-checks/gr-checks-view.js
+++ b/gr-checks/gr-checks-view.js
@@ -165,6 +165,7 @@
     },
 
     _handleRevisionUpdate(revision) {
+      if (!revision) return;
       this._currentPatchSet = revision._number;
     },