Call to a member function roles() on null Laravel

I'm making user roles function in Laravel and everything has worked fine and from nowhere I have error: call to a member function roles() or null.

This is my html from where I'm sending request:

<td><input type="checkbox"  name="role_user"></td>
<td><input type="checkbox"  name="role_author"></td>
<td><input type="checkbox"  name="role_admin"></td>

Then there are function where I'm making relationship between model User and model Role, and functions where I'm checking the roles:

 public function roles(){
        return $this->belongsToMany('App\Role','role_user','user_id','role_id');
    }

  public function hasAnyRole($roles){
        if(is_array($roles)){
            foreach ($roles as $role){
                if($this->hasRole($role)){
                    return true;
                }
            }
        }else{
            if($this->hasRole($roles)){
                return false;
            }
        }
        return false;
    }


    public function hasRole($role){
        if($this->roles->where('name',$role)->first()){
            return true;
        }
        return false;
    }

And in the end the function where I'm assing new roles:

public function postAdminAssignRoles(Request $request)
    {
        $user = User::where('email', $request['email'])->first();
        $user->roles()->detach();
        if ($request['role_user']) {
            $user->roles()->attach(Role::where('name', 'User')->first());
        }
        if ($request['role_author']) {
            $user->roles()->attach(Role::where('name', 'Author')->first());
        }
        if ($request['role_admin']) {
            $user->roles()->attach(Role::where('name', 'Admin')->first());
        }
        return redirect()->back();
    }

I have no idea why this is not working now, because everything was working fine till now. Any ideas?

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