Django An invalid form control with name='' is not focusable - html

Question

I created a Django form that have dropdown in one of the field. When I tried to submit the form without selecting the dropdown item, the form is not submitted because I set in my code that it won't submit unless the form valid using is_valid().
However, it was not showing the validation error message. In the browser console, the is an error message An invalid form control with name='start_position' is not focusable.
I have read some questions about this and they said that it happened because there is a hidden field. After I inspect element, it is true that div class='start_position' is hidden.
However, I don't know how to fix it in Django.
This is the code that create dropdown field.
class LocationForm(forms.Form):
start_position = forms.ModelChoiceField(
required=True,
label="Start Position",
queryset=Location.objects.all()
)
This is the rendered HTML
<div class="formColumn form-group col-md-6 mb-0">
<div id="div_id_start_position" class="form-group">
<label for="id_start_position" class="col-form-label requiredField">
Start Position<span class="asteriskField">*</span>
</label> <div class="">
<select name="start_position" class="select form-control" required="" id="id_start_position" style="display: none;">
<option value="" selected="">---------</option>
<option value="78">Loc 78</option>
<option value="79">Loc 79</option>
<option value="80">Loc 80</option>
</select><div class="nice-select select form-control" tabindex="0">
<span class="current">Loc 78</span>
<ul class="list">
<li data-value="" class="option">---------</li>
<li data-value="78" class="option selected">Loc 78</li>
<li data-value="79" class="option">Loc 79</li>
<li data-value="80" class="option">Loc 80</li>
</ul></div>
</div>
</div>
I want the validation message show like in the other field (I tried DateField and the validation works normally).