Steps to reproduce:

Create html widget:

```
 

{% set source %}

<selectclass="ibexa-input">
<optionvalue="DE">DE</option>
<optionvalue="US">US</option>
<optionvalue="NO">NO</option>
<optionvalue="PL">PL</option>
</select>

{% endset %}

{% set choices = [

{ value: "DE", label: "DE" }

,

{ value: "US", label: "US" }

,

{ value: "NO", label: "NO" }

,

{ value: "PL", label: "PL" }

] %}
{% include '@ezdesign/ui/component/dropdown.html.twig' with

{ source: source, choices: choices, multiple: true, custom_form: true, }

%}
```
Expected behaviour:
Everything works
Actual behaviour:
Dropdown throws error