backuptask: fix crash when accessing stats of old backups

src/backuptask.js

@@ -283,7 +283,8 @@ async function backupBox(backupSite, appBackupsMap, tag, options, progressCallba
 
     debug(`backupBox: rotating box snapshot of ${backupSite.id} to id ${remotePath}`);
 
-    stats.aggregated = Array.from(appBackupsMap.values()).reduce((acc, s) => ({
+    // stats object might be null for stopped/errored apps from old versions
+    stats.aggregated = Array.from(appBackupsMap.values()).filter(s => !!s).reduce((acc, s) => ({
         fileCount: acc.fileCount + s.fileCount,
         size: acc.size + s.size,
         startTime: Math.min(acc.startTime, s.startTime),
@@ -373,7 +374,7 @@ async function backupAppWithTag(app, backupSite, tag, options, progressCallback)
     if (!apps.canBackupApp(app)) { // if we cannot backup, reuse it's most recent backup
         const lastKnownGoodAppBackup = await backups.getLatestInTargetByIdentifier(app.id, backupSite.id);
         if (lastKnownGoodAppBackup === null) return null; // no backup to re-use
-        return lastKnownGoodAppBackup.id;
+        return { id: lastKnownGoodAppBackup.id, stats: lastKnownGoodAppBackup.stats };
     }
 
     const { stats, integrity } = await uploadAppSnapshot(backupSite, app, progressCallback);