{% if Category is not null %}
    
    {% else %}
        {% set cate_childs = Category.getDescendants %}
        {% if cate_childs %}
        {% set h = Category.hierarchy + 1 %}
        <div id="cate_list">
        <h2 class="cate_h2">{{ subtitle }}　カテゴリー一覧</h2>
        <ul class="cate_list">
        {% for cate in cate_childs %}
            {% if h == cate.hierarchy %}
            <li><a href="/products/list?category_id={{ cate.id }}">{{ cate.name }}</a></li>
            {% endif %}
        {% endfor %}
        </ul>
        </div>
        {% endif %}
        
    {% endif %}

Impossible to access an attribute ("getDescendants") on a null variable.