title: Bootstrap 4 Custom Checkbox Not Shown
body: <p>i want to use bootstrap 4 custom control for checkbox, after reading the documentation i wrote this, but the checkbox is not shown, what am i doing wrong?
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  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;link href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" rel="stylesheet"/&gt;
  &lt;div class="card-body"&gt;&lt;label&gt;Add Productc Varian for &lt;/label&gt;
    &lt;div class="form-group"&gt;&lt;label for="formGroupExampleInput"&gt;Code&lt;/label&gt;&lt;input type="text" class="form-control" placeholder="Enter Name"&gt;&lt;/div&gt;
    &lt;div class="form-group"&gt;&lt;label for="formGroupExampleInput"&gt;Name&lt;/label&gt;&lt;input type="text" class="form-control" placeholder="Enter Name"&gt;&lt;/div&gt;
    &lt;div class="form-group"&gt;&lt;label for="formGroupExampleInput2"&gt;Description&lt;/label&gt;&lt;textarea class="form-control" placeholder="Description"&gt;&lt;/textarea&gt;&lt;/div&gt;
    &lt;div class="form-group"&gt;
      &lt;div class="custom-control custom-checkbox"&gt;&lt;label for="formGroupExampleInput2"&gt;Default&lt;/label&gt;&lt;input type="checkbox" class="custom-control-input" id="customControlValidation1"&gt;&lt;/div&gt;
    &lt;/div&gt;
    
  &lt;hr&gt;
  &lt;div class="form-group"&gt;
    &lt;div class="col-12"&gt;&lt;button type="button" class="btn btn-default float-right"&gt;Add&lt;/button&gt;&lt;button type="button" class="btn btn-danger float-right"&gt;Cancel&lt;/button&gt;&lt;/div&gt;
  &lt;/div&gt;
  &lt;/div&gt;</code></pre>
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