CKEditor: Assets chooser opens in background and can't be accessed

Our customer has found same bug than MGNLUI-5018 when using CKEditor from a content app form instead of within a component dialog.

Steps to reproduce using our demo (tours app):

1. Set images to true in /ckeditor/config-magnolia.js (line l14) in order to be able to use image browser plugin
2. In configuration app, add configJsFile property with value /.resources/ckeditor/config-magnolia.js to /modules/tours/apps/tours/subApps/detail/editor/form/tabs/tour/fields/body
3. When opening the edit tour form and press image button in the CKEditor, the asset chooser will be placed background and won't be editable:
   

This doesn't happen when CKEditor is placed within a dialog, for example when using it within a component in pages app (that's what was fixed in MGNLUI-5018)