Error in slick.js: "Uncaught TypeError: Cannot read property 'add' of null"

I used slick js for slider view of image.

Here is my code.

<div class="slider_wrap add-remove">
    <%= f.fields_for :images do |image_form| %>
      <%#= render 'images_fields', :f => image_form %>
        <div>
          <%= image_tag image_form.object.picture.url,:class=>"drop_down_link even img_prev" %>
        </div>
        <div class="image_upload_div">
          <div class="image-upload">
            <label>
              <i class="fa fa-cloud-upload">
                <%= image_form.file_field :picture ,:'data-role'=>"none",:onchange=>"readURL(this);" , :accept => 'image/jpeg , image/png' %>
              </i>
            </label>
          </div>
        </div>
    <% end %>
    <%= f.link_to_add "Add a picture", :images ,:id=>"add_pic" ,:class=>"js-add-slide", :style=>"display: none;"%>
</div>

<script>
function silder(){
    slideIndex = 0;
      $('.add-remove').slick({
        slidesToShow: 2,
        slidesToScroll: 2
      });
      $('.js-add-slide').on('click', function() {
        $('.add-remove').slick('slickAdd');
      });

      $('.js-remove-slide').on('click', function() {
        $('.add-remove').slick('slickRemove');
      });
});
function readURL(input) {
  if (input.files && input.files[0]) {
    var reader = new FileReader();
    reader.onload = function (e) {
    $('.img_prev').last()
      .attr('src', e.target.result)
    };

    reader.readAsDataURL(input.files[0]);

    setTimeout(function(){
      $('#add_pic').trigger('click');
      silder();
    }, 100);
  }
}
</script>

Now with this code I got slider working, but its not coming proper I am getting error:

Uncaught TypeError: Cannot read property 'add' of null

That's due to calling init twice. This works without error:

$(".slider").not('.slick-initialized').slick()

Also, "silder" looks like a typo.

Relying on a timeout is also error-prone. Unfortunately, Internet Explorer uses a different event to tell you when the HTML elements and JS libraries have been loaded. There are many libraries to avoid the 100 or so lines of cross-browser code, but the popular and relatively small jQuery solves the timing issue like this:

$(function() {
  // Handler for .ready() called. Put the Slick Slider etc. init code here.
})