Revisit all FIXME's and TODO's inside code

[Florian-Schoenherr]

Coming from #160, we need to find out what parts of the code still really need to be fixed/done. Main priority is what was mentioned in the issue: what was this line, where null was collector before supposed to do?

And while I'm at it, I would like to also fix the other TODO/FIXME's around the code. Maybe pointing to new issues. But I'm not certain which parts of it are still relevant. I hope that people who already worked on some of the non-ui code can help me out. As far as I know, @pflueras did ES-CQRS, so you will probably know a lot about the code base. If anyone else would know more about certain TODO/FIXME's, please @ them here.