Upgrade 10.3.0 to 10.4.0 crashes during opening the app

Upon opening my 10.3.0 app in MxStudioPro 10.4.0 and accepting Mendix to convert the app to 10.4.0, it runs in this error:   My app's marketplace modules are up-to-date and /userlib is cleaned from old jar-files.   Editted: It does have to do with the app's styling. Still to find out the exact cause.
1 answers

in some instances custom UI modules have a empty settings.json file. Since Mendix 10.4 this generates a NULL error and compiling is no longer working. The solution is to add some empty brackets in the settings.json, as it can't be empty:

{ }

Now their is an empty awnser instead of a NULL error and compiling is working again.