Hello Ringo,
What we did in previous examples (and discussed in previous discussion where your question is originated from) is: we embed SAPUI5 elements into a WebUI view.
What you're trying to do is a little bit different. But interesting and I will try to have a look at your issue. No promises though.
As far as I know SAPUI5 is not yet supported in WebUI. As of now in SAP User Interface Technologies - Road Map there is a mention of "HTML5 Islands" for WebUI in planned innovations. Probably this feature will solve your issue. And in general will let embed SAPUI5/Fiori-alike apps into WebUI easily.