Sergio,
Just open the main config.json and find the attributetable widget and change the closable attribute from true to false:
{
"position": {
"left": 60,
"top": 53,
"relativeTo": "map"
},
"placeholderIndex": 1,
"id": "_28",
"name": "AttributeTable",
"version": "2.2",
"featureActions": [
{
"name": "ShowRelatedRecords",
"uri": "actions/ShowRelatedRecordsFeatureAction"
},
{
"name": "ViewInTable",
"uri": "actions/ViewInAttributeTableFeatureAction"
}
],
"uri": "widgets/AttributeTable/Widget",
"closeable": false
},