Add null checks for invalid API response

index.js

@@ -16,10 +16,12 @@ bittrex.options({
 function sellIfTargetReached() {
   bittrex.getticker( { market : 'BTC-LSK' }, function(lskBtcData, err ) {
 
+    if(lskBtcData) {
       var lskBtcPrice = lskBtcData.result.Last;
 
       bittrex.getticker({market: 'USDT-BTC'}, function (btcUsdData, err) {
 
+        if(btcUsdData) {
           var btcUsdtPrice = btcUsdData.result.Last;
 
           lskUsdPrice = parseFloat(lskBtcPrice * btcUsdtPrice);
@@ -49,7 +51,9 @@ function sellIfTargetReached() {
 
             process.exit();
           }
+        }
       });
+    }
   });
 }