We do need a validate method, but it needed a fix.

dist/converse.js

@@ -50971,9 +50971,15 @@ _converse_headless_converse_core__WEBPACK_IMPORTED_MODULE_5__["default"].plugins
       validate(attrs, options) {
         const _converse = this.__super__._converse;
 
-        if (_converse.view_mode === 'embedded' && attrs.type === _converse.CONTROLBOX_TYPE) {
+        if (attrs.type === _converse.CONTROLBOX_TYPE) {
+          if (_converse.view_mode === 'embedded') {
             return 'Controlbox not relevant in embedded view mode';
           }
+
+          return;
+        }
+
+        return this.__super__.validate.apply(this, arguments);
       },
 
       initialize() {

src/converse-controlbox.js

@@ -145,6 +145,17 @@ converse.plugins.add('converse-controlbox', {
         },
 
         ChatBox: {
+            validate (attrs, options) {
+                const { _converse } = this.__super__;
+                if (attrs.type === _converse.CONTROLBOX_TYPE) {
+                    if (_converse.view_mode === 'embedded')  {
+                        return 'Controlbox not relevant in embedded view mode';
+                    }
+                    return;
+                }
+                return this.__super__.validate.apply(this, arguments);
+            },
+
             initialize () {
                 if (this.get('id') === 'controlbox') {
                     this.set({'time_opened': moment(0).valueOf()});