Fix error in view-padding example

An error occured when the buttons are clicked before the source is loaded.

examples/view-padding.js

@@ -50,8 +50,9 @@ const map = new Map({
   view: view,
 });
 
-const zoomtoswitzerland = document.getElementById('zoomtoswitzerland');
-zoomtoswitzerland.addEventListener(
+vectorLayer.getSource().on('featuresloadend', function () {
+  const zoomtoswitzerland = document.getElementById('zoomtoswitzerland');
+  zoomtoswitzerland.addEventListener(
     'click',
     function () {
       const feature = source.getFeatures()[0];
@@ -59,10 +60,10 @@ zoomtoswitzerland.addEventListener(
       view.fit(polygon);
     },
     false
-);
+  );
 
-const centerlausanne = document.getElementById('centerlausanne');
-centerlausanne.addEventListener(
+  const centerlausanne = document.getElementById('centerlausanne');
+  centerlausanne.addEventListener(
     'click',
     function () {
       const feature = source.getFeatures()[1];
@@ -70,4 +71,5 @@ centerlausanne.addEventListener(
       view.setCenter(point.getCoordinates());
     },
     false
-);
+  );
+});