[Glitch] Do not try to update an undefined video element

Port 7840c6b75b to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

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

@@ -441,6 +441,10 @@ class Video extends PureComponent {
   };
 
   _syncVideoToVolumeState = (volume = null, muted = null) => {
+    if (!this.video) {
+      return;
+    }
+
     this.video.volume = volume ?? this.state.volume;
     this.video.muted = muted ?? this.state.muted;
   };