The error seems to be located at line 18 of the controller :

Which corresponds to this line 

    item.querySelector("input[body*='_destroy']").value = 1

<%= simple_form_for ([@survey]) do |f| %>
  <%= f.input :name, label:"Survey name"%>
  <div data-controller='nested', data-nested-index-value='QUESTION_RECORD'>
    <template data-nested-target="template">
      <%= f.simple_fields_for :questions, Question.new, child_index: 'QUESTION_RECORD' do |question| %>
        <%= render "question_fields", f: question %>
      <% end %>
    </template>

    <%= f.simple_fields_for :questions do |question| %>
      <%= render "question_fields", f: question %>
    <% end %>

    <!-- Inserted elements will be injected before that target. -->
    <div data-nested-target="add_item">
      <%= f.button :submit, "Add question", data: {action:"nested#add_association"} %>
    </div>
  </div>
  <%= f.submit 'Create survey' %>
<% end %>

<div class="nested-wrapper" data-new-record="<%= f.object.new_record? %>" data-controller='dynamic-select'>
  <%= f.input :category do %>
    <% f.select :category, Question.category_select, selected: f.object.category,
    'data-dynamic-select-target': 'select',
    'data-action': 'dynamic-select#selected' %>
  <% end %>
  <%= f.input :body, label:"Question" %>
  <%= f.button :submit, "Delete question", data: {action:"nested#remove_association"} %>
  <%= f.hidden_field :_destroy %>

  <div data-controller='nested', data-nested-index-value='ANSWER_RECORD' data-dynamic-select-target='choice'>
    <template data-nested-target="template">
      <%= f.simple_fields_for :answers, Answer.new, child_index: 'ANSWER_RECORD' do |answer| %>
        <%= render "answer_fields", f: answer %>
      <% end %>
    </template>

    <%= f.simple_fields_for :answers do |answer| %>
      <%= render "answer_fields", f: answer %>
    <% end %>

    <div data-nested-target="add_item">
        <%= f.button :submit, "Add answer", data: {action:"nested#add_association"} %>
    </div>
  </div>
  <div data-controller="nested-form" data-dynamic-select-target='long'>
  </div>
</div>

<div class="nested-wrapper" data-new-record="<%= f.object.new_record? %>">
  <%= f.hidden_field :id %>
  <%= f.input :body, label:"Answer" %>
  <%= f.button :submit, "Delete answer", data: {action:"nested#remove_association"} %>
  <%= f.hidden_field :_destroy %>
</div>