Fix of challenge listing

Format of MM objects returned by the backend has changed slightly, and
it broke our frontend code. This is the fix.

src/shared/reducers/challenge-listing.js

@@ -84,8 +84,8 @@ export function normalizeMarathonMatch(challenge) {
     currentPhases: allphases.filter(phase => phase.phaseStatus === 'Open'),
     communities: new Set([COMMUNITY.DATA_SCIENCE]),
     currentPhaseName: endTimestamp > Date.now() ? 'Registration' : '',
-    numRegistrants: challenge.numRegistrants[0],
-    numSubmissions: challenge.userIds.length,
+    numRegistrants: challenge.numRegistrants ? challenge.numRegistrants[0] : 0,
+    numSubmissions: challenge.userIds ? challenge.userIds.length : 0,
     platforms: '',
     prizes: [0],
     registrationOpen: endTimestamp > Date.now() ? 'Yes' : 'No',