A tree builder without a root node is deprecated since Symfony 4.2 and will not be supported anymore in 5.0.

change text for variable of internationalization "save"

add validation for range label
 <div class="col-sm-1">
                                    <label class="col-m-3 control-label textalingleft">
                                        {% if form.vars.data.range == 'minutes' %}
                                            {{'crontask.minutes'|trans}}
                                        {% endif %}
                                        {% if form.vars.data.range == 'hours' %}
                                            {{'crontask.hours'|trans}}
                                        {% endif %}
                                        {% if form.vars.data.range == 'days' %}
                                            {{'crontask.days'|trans}}
                                        {% endif %}
                                    </label>
                                </div>