Only preload emoji after the window is fully loaded

js/background.js

@@ -37,6 +37,7 @@
   );
 
   // Load these images now to ensure that they don't flicker on first use
+  window.Signal.EmojiLib.preloadImages();
   const images = [];
   function preload(list) {
     for (let index = 0, max = list.length; index < max; index += 1) {