LATEST UPDATE: @AcrobatEpee picked the solution from the universe and brought it to earth. You can see an example image of the workaround or solution in another more specific thread:
I give up - can any great guy help me? Otherwise no coding for me today 
UPDATE: Still coding 
I recommend everyone who wants to add an item to a list to use listviewer instead of list variable. just set the height to 0 (zero). Then the listitems can be used instead of a list-variable. This workaround works for me in my project, which does a SQL like sorting of objects (sort objects by price / likes / creationdate)
… but beware the bug, that adding to list for listviewer only works for > 1 item , otherwise (length of list = 1) you have to assign the list item with a different block