JavaScript bug when editing exercise sets and first set is a "REST" ?

I'm not the hottest JS coder, so excuse me if I am misunderstanding this, but when I sync a Strength activity that includes exercise sets where the first set is a REST (I think I did this because I paused the activity before actually starting), the web interface crashes (and so does the Android app);

In https://connect.garmin.com/web-react/static/js/backbone/models/activity/RepCountingActivity.js line 154 -

 getConsolidatedJSON: function() {
   var allSets = this.get('exerciseSets'),
   restTime = 0,
   sets = [],
   that = this;

   _.each(allSets, function(set) {
     if (set.setType === 'REST') {
       if (!sets[sets.length - 1].rest) {               <====== this is the error line
         sets[sets.length - 1].rest = 0;
       }

       sets[sets.length - 1].rest += set.duration;
     } else {
       set.exerciseType = that.getExerciseType(set.exercises);
       sets.push(set);
     }
   });

 return sets;
 },

Then, with this data set (truncated!):

{"activityId":4172875329,"exerciseSets":[{"exercises":[],"duration":4.416,"repetitionCount":null,"weight":null,"setType":"REST","startTime":"2019-10-19T12:46:29.0","wktStepIndex":null},{"exercises":[{"category":"UNKNOWN","name":null,"probability":99.609375},{"category":"CURL","name":null,"probability":0.0},{"category":"BENCH_PRESS","name":null,"probability":0.0}],"duration":26.901,"repetitionCount":20,"weight":39000.0,"setType":"ACTIVE","startTime":"2019-10-19T12:46:33.0","wktStepIndex":null},{"exercises":[],"duration":59.56,"repetitionCount":null,"weight":null,"setType":"REST","startTime":"2019-10-19T12:47:00.0","wktStepIndex":null},{"exercises":[{"category":"UNKNOWN","name":null,"probability":99.609375},{"category":"CURL","name":null,"probability":0.0},{"category":"BENCH_PRESS","name":null,"probability":0.0}],"duration":35.358,"repetitionCount":15,"weight":45000.0,"setType":"ACTIVE","startTime":"2019-10-19T12:47:59.0","wktStepIndex":null},

the when sets[] is empty, sets.length - 1 is < 0 and you get an error:

RepCountingActivity.js:154 Uncaught TypeError: Cannot read property 'rest' of undefined
 at RepCountingActivity.js:154
 at Function.v.each.v.forEach (underscore.js:153)
 at r.getConsolidatedJSON (RepCountingActivity.js:152)
 at renderSets (EditRepCountingActivityView.js:83)
 at render (EditRepCountingActivityView.js:54)
 at r.showRepCountingElements (EditActivityPageView.js:640)
 at render (EditActivityPageView.js:507)
 at r.<anonymous> (PageView.js:89)
 at A (underscore.js:701)
 at r.i [as render] (underscore.js:733)

Someone, assuming my JS is not completely mistaken (and it could be) needs to check for the empty set first.

When my activities don't start with a "REST" then all seems OK.