Error while combining multiple category Drop Down ,Error `id' for nil:NilClass

I have two category set for a job and have set category id to them, Now how to view multiple Drop Down when they are selected. And also which one is better multiple checkbox or Drop Down ??

index.html.haml
%body
%nav.navbar.navbar-job
        .container
                %ul.nav.navbar-nav
                        %li= link_to "All Creative Jobs", jobs_path
                        - Category2.all.each do |category2|
                                %li= link_to category2.name, jobs_path(category2: category2.name)
                        
                        %li.dropdown
                                %a.dropdown-toggle{"data-toggle" => "dropdown","role" => "button", "aria-expanded" => "false"}
                                %ul.dropdown-menu
                                        - Jobcategory.all.each do |jobcategory|
                                                %li= link_to jobcategory.name, jobs_path(jobcategory: jobcategory.name)
class JobsController < ApplicationController
        before_action :find_job, only: [:show, :edit, :update, :destroy]
        before_action :authenticate_user!,except:[:index]


        def index
                if params[:jobcategory].blank? && params[:category2].blank?
                        @jobs = Job.all.order("created_at DESC")
                else
                        @jobcategory_id = Jobcategory.find_by(name: params[:jobcategory]).id
                        @category2_id = Category2.find_by(name: params[:category2]).id
                        @jobs = Job.where(["jobcategory = ? and category2 = ?", @jobcategory_id, @category2_id]).order("created_at DESC")

                end
        end