Call to a member function addvoter() on null

On Laravel 5.4.1 I'm always getting this error when using ajax

FatalThrowableError in avController.php line 53: Call to a member function addvoter() on null

but it seems to work perfectly without ajax. I think the problem starts from $request->user()->addvoter()->save($voter); which is my line 53 but I don't understand it;

/**
 * Store a newly created resource in storage.
 *
 * @param  \Illuminate\Http\Request  $request
 * @return \Illuminate\Http\Response
 */
public function store(Request $request)
{
    if ($request->isMethod('post')){ 
           $voter = new voter();
           $voter->FirstName = $request->fname;
           $voter->LastName= $request->lname;
           $voter->Email = $request->email;
           $voter->PhoneNumber = $request->phone;
           $voter->Gender = $request->optradio;
           $voter->StudentID = $request->sID;
           $voter->Level = $request->level;
           $voter->Course = $request->course;
           $voter->Stream = $request->stream;
           $voter->Nationality = $request->nat;
           $voter->ProfileImg = $request->image;     

           $request->user()->addvoter()->save($voter);

          $response = array(
               'fname' => $request->fname,
               'lname' => $request->lname, 
               'msg' => 'Setting created successfully',
           );
             return response()->json($response);
       }
       return response()->json(['response' => 'This is get method']);
}

my user model

use Notifiable;

/**
 * The attributes that are mass assignable.
 *
 * @var array
 */
protected $fillable = [
    'name', 'email', 'password',
];

/**
 * The attributes that should be hidden for arrays.
 *
 * @var array
 */
protected $hidden = [
    'password', 'remember_token',
];

public function addcandidate(){
    return $this->hasMany('elpebi\addcandidate');
} 
 public function addstcandidate(){
    return $this->hasMany('elpebi\stcandidate');
}
public function addvoter(){
    return $this->hasMany('elpebi\voter');
}
public function newelection(){
    return $this->hasMany('elpebi\election');
}

my voter model

protected $fillable = ['FirstName', 'LastName', 'Email', 
                       'Gender', 'StudentID', 'Level', 'Course',       
                       'Stream', 'Nationality', 'ProfileImg' ];
public function user(){
    return $this->belongsTo('elpebi\User');
}

via bil mason