Fix renderChildNodes/refresh errors when configuring/saving a form layout

After creating a custom document and the configuring it's Create layout a lot of errors in the console are logged one of them being: 

 Uncaught (in promise) TypeError: Cannot read property 'lastChild' of null
    at nuxeo-view-designer.bundle.js:5908
    at new Promise (<anonymous>)
    at HTMLElement._renderChildNodes (nuxeo-view-designer.bundle.js:5908)
    at nuxeo-view-designer.bundle.js:5908

How to reproduce:

• Go to studio and in Modeler create new custom document
• go to Designer and configure the create layout of that new document
• some errors are logged in the browser console

console errors image attached