Fix play/pause toggle bug

Sometimes, while casting, WT thinks the video isn't visible and the play/pause toggle stops working

renderer/controllers/playback-controller.js

@@ -54,8 +54,8 @@ module.exports = class PlaybackController {
 
     // force rerendering if window is hidden,
     // in order to bypass `raf` and play/pause media immediately
-    if (!state.window.isVisible) {
-      var mediaTag = document.querySelector('video,audio')
+    var mediaTag = document.querySelector('video,audio')
+    if (!state.window.isVisible && mediaTag) {
       if (state.playing.isPaused) mediaTag.play()
       else mediaTag.pause()
     }