[FIX] web: can reopen ModelField selector widget

Before this fix, in studio when the user select the field then focus out
then focus in and selected a child field, an exception is triggered because
the last page is visible but the data are removed.

backport of 12.0's 9eef25ae0a

opw-1928453
closes #30277

addons/web/static/src/js/widgets/model_field_selector.js

@@ -257,7 +257,6 @@ var ModelFieldSelector = Widget.extend({
 
         if (this.dirty) {
             this.dirty = false;
-            this.pages = this.pages.slice(0, this.chain.length || 1);
             this.trigger_up("field_chain_changed", {chain: this.chain});
         }
     },