fixes

Author: jogibear9988

packages/web-component-designer/src/elements/widgets/designerView/designerCanvas.ts

@@ -806,7 +806,7 @@ export class DesignerCanvas extends BaseCustomWebComponentLazyAppend implements
  if (this.children.length == 1 && this.children[0] instanceof HTMLSlotElement) {
  children = <any>this.children[0].assignedElements();
  }
- if (children.length > 0) {
+ if (children?.length > 0) {
  const parser = this.serviceContainer.getLastServiceWhere('htmlParserService', x => x.constructor == DefaultHtmlParserService) as DefaultHtmlParserService;
  this.addDesignItems(parser.createDesignItems(children, this.serviceContainer, this.instanceServiceContainer));
  }