[Glitch] Fix error when closing a playing audio or video modal

Port ddd69f3ae9 to glitch-soc

Signed-off-by: Thibaut Girka <thib@sitedethib.com>

app/javascript/flavours/glitch/features/audio/index.js

@@ -283,6 +283,8 @@ class Audio extends React.PureComponent {
 
   _renderCanvas () {
     requestAnimationFrame(() => {
+      if (!this.audio) return;
+
       this.handleTimeUpdate();
       this._clear();
       this._draw();

app/javascript/flavours/glitch/features/video/index.js

@@ -195,6 +195,8 @@ class Video extends React.PureComponent {
 
   _updateTime () {
     requestAnimationFrame(() => {
+      if (!this.video) return;
+
       this.handleTimeUpdate();
 
       if (!this.state.paused) {