message: Add a null guard for message event previews
diff: @@ -30,6 +30,8 @@ export class MessageEventPreview implements IPreview {
  eventContent = event.getContent()['m.new_content'];
  }
  + if (!eventContent || !eventContent['body']) return null; // invalid for our purposes
  +
  let body = (eventContent['body'] || '').trim();
  const msgtype = eventContent['msgtype'];
  if (!body || !msgtype) return null; // invalid event, no preview
  