Fix(nav): Protect against missing data

common/app/redux/load-current-challenge-saga.js

@@ -63,6 +63,10 @@ export function loadCurrentChallengeSaga(actions, getState) {
           challengeIdToName[ currentChallengeId ]
         ];
       }
+      // challenge data may not be set yet.
+      if (!finalChallenge.id) {
+        return Observable.empty();
+      }
       if (finalChallenge.id === currentlyLoadedChallengeId) {
         // don't reload if the challenge is already loaded.
         // This may change to toast to avoid user confusion

common/app/redux/selectors.js

@@ -22,12 +22,17 @@ export const firstChallengeSelector = createSelector(
     ) {
       return {};
     }
-    return challengeMap[
-      blockMap[
-        superBlockMap[
-          superBlocks[0]
-        ].blocks[0]
-      ].challenges[0]
-    ];
+    try {
+      return challengeMap[
+        blockMap[
+          superBlockMap[
+            superBlocks[0]
+          ].blocks[0]
+        ].challenges[0]
+      ];
+    } catch (err) {
+      console.error(err);
+      return {};
+    }
   }
 );