When I try to start my application, the layout is destroyed and there seems to be a conflict in the npm modules, as the console shows:
Uncaught DOMException: Failed to execute 'define' on 'CustomElementRegistry': the name "dom-module" has already been used with this registry
at eval (webpack-internal:///../node_modules/@polymer/paper-dropdown-menu/node_modules/@polymer/polymer/lib/elements/dom-module.js?babel-target=es6:181:16)
at Module.../node_modules/@polymer/paper-dropdown-menu/node_modules/@polymer/polymer/lib/elements/dom-module.js?babel-target=es6 (http://localhost:8088/VAADIN/build/vaadin-bundle-819c19c73ff74dc82dd2.cache.js:625:1)
at __webpack_require__ (http://localhost:8088/VAADIN/build/vaadin-bundle-819c19c73ff74dc82dd2.cache.js:20:30)
at eval (webpack-internal:///../node_modules/@polymer/paper-dropdown-menu/node_modules/@polymer/polymer/lib/utils/style-gather.js?babel-target=es6:10:81)
at Module.../node_modules/@polymer/paper-dropdown-menu/node_modules/@polymer/polymer/lib/utils/style-gather.js?babel-target=es6 (http://localhost:8088/VAADIN/build/vaadin-bundle-819c19c73ff74dc82dd2.cache.js:1021:1)
at __webpack_require__ (http://localhost:8088/VAADIN/build/vaadin-bundle-819c19c73ff74dc82dd2.cache.js:20:30)
at eval (webpack-internal:///../node_modules/@polymer/paper-dropdown-menu/node_modules/@polymer/polymer/lib/mixins/element-mixin.js?babel-target=es6:8:80)
at Module.../node_modules/@polymer/paper-dropdown-menu/node_modules/@polymer/polymer/lib/mixins/element-mixin.js?babel-target=es6 (http://localhost:8088/VAADIN/build/vaadin-bundle-819c19c73ff74dc82dd2.cache.js:733:1)
at __webpack_require__ (http://localhost:8088/VAADIN/build/vaadin-bundle-819c19c73ff74dc82dd2.cache.js:20:30)
at eval (webpack-internal:///../node_modules/@polymer/paper-dropdown-menu/node_modules/@polymer/polymer/lib/legacy/legacy-element-mixin.js?babel-target=es6:4:82)
It seems that the polymer version could be the problem:
package-lock.json states that
"@polymer/paper-dropdown-menu": {
"version": "3.1.0",
"dependencies": {
"@polymer/polymer": {
"version": "3.3.1",
and package.json states
"dependencies": {
"@polymer/polymer": "3.2.0",