PB-500 make column group compatible to pre-existing content - set default appearance if it is not already set

Author: paras89

app/code/Magento/PageBuilder/view/adminhtml/web/js/content-type/column-group/preview.js

@@ -157,6 +157,8 @@ export default class Preview extends PreviewCollection {
                 }
             });
 
+        const appearance = this.contentType.dataStore.get("appearance") ? this.contentType.dataStore.get("appearance") : 'default';
+        this.contentType.dataStore.set("appearance", appearance);
         this.contentType.dataStore.set("non_empty_column_count", numCols - numEmptyColumns);
         this.contentType.dataStore.set("max_grid_size", getMaxGridSize());
         this.contentType.dataStore.set("initial_grid_size", this.contentType.dataStore.get("grid_size"));