jLuger.de - Firefox error: No element found

Today I've got a strange error message in my JavaScript console in Firefox: No element found. It appeared while a GET that was done in the callback function of a POST and had the first line and first character of my script file as source.

To make a long story short: It wasn't the GET but the POST. The POST called a method in the Backend that didn't return a value. That turned the response type to xml. I've changed my method to return a response like this:

		return Response.noContent().type(MediaType.APPLICATION_JSON).build();

Now the response type is json and the error appears no longer.