500 Internal Server Error in local install

500 Internal Server Error

Oops, An exception has been thrown during the rendering of a template (“array_merge(): Argument #1 is not an array”) in “forms/fields/menu/item.html.twig” at line 6..

This error code pops up when I try to assign a menu to the “menu particle” in the Helium theme.

PS: I am a newbie at Gantry framework, just started my first project. Please help

Theme: Helium (v5.3.2 / g5_helium)
Gantry Version 5.3.2
Timber Version 1.1.2

The code from item.html.twig

{% extends 'forms/fields/select/selectize.html.twig' %}

{% block options %}
    {{ parent() }}
    {% if gantry.menu is not null %}
        {% for group, items in gantry.menu.getGroupedItems() %}
            {% if items|length %}
            <optgroup label="{{ group|capitalize }}">
            {% for key, item in items %}
            <option
                    {# required attribute structures #}
                    {% if key == value %}selected="selected"{% endif %}
                    value="{{ key }}"
                    {# non-gloval attribute structures #}
                    {% if field.options.disabled in ['on', 'true', 1] %}disabled="disabled"{% endif %}
                    >{{ item.spacing|raw }}{{ item.label }}</option>
            {% endfor %}
            </optgroup>
            {% endif %}
        {% endfor %}
    {% endif %}
{% endblock %}

Note: I have been using Gantry 5 hydrogen theme and I did not experience this problem.