Invalid/Not existed username handle in API ShowUserController

[ganuonglachanh]

Bug Report

Current Behavior
spinning icon with error popup when go to url with invalid/not existed usename

Steps to Reproduce

1. Go to https://discuss.flarum.org/u/notexisteduser
2. See error

Expected Behavior
A user not found handler, not a spinning icon with error popup

Environment

• Flarum version: beta 0.9

Possible Solution

    protected function data(ServerRequestInterface $request, Document $document)
    {
        $id = Arr::get($request->getQueryParams(), 'id');
        if (! is_numeric($id)) {
            $id = $this->users->getIdForUsername($id);
        }
        $actor = $request->getAttribute('actor');
        if ($actor->id == $id) { 
//should be === when $actor->id = 0 and $id = null
            $this->serializer = CurrentUserSerializer::class;
        }
        return $this->users->findOrFail($id, $actor);
 //-> need better handler when actor is guest and $id = null, save some DB queries
    }