Fix media modal crashing when media has no blurhash (#15229)

app/javascript/mastodon/features/ui/components/media_modal.js

@@ -221,10 +221,13 @@ class MediaModal extends ImmutablePureComponent {
   _sendBackgroundColor () {
     const { media, onChangeBackgroundColor } = this.props;
     const index = this.getIndex();
-    const backgroundColor = decodeRGB(decode83(media.getIn([index, 'blurhash']).slice(2, 6)));
+    const blurhash = media.getIn([index, 'blurhash']);
 
+    if (blurhash) {
+      const backgroundColor = decodeRGB(decode83(blurhash.slice(2, 6)));
       onChangeBackgroundColor(backgroundColor);
     }
+  }
 
   componentWillUnmount () {
     window.removeEventListener('keydown', this.handleKeyDown);