Changeset 543f2876071b introduces a bug that causes simpy.io/mosaik to hang forever.

Created by: sscherfke

@BengtOFFIS, @gerstejo and I found out that changeset 543f2876071b introduced something that causes simpy.io to hang forever (it hangs here).

You can run the mosaik demo to reproduce this problem. We have no better example or a regression test yet, but only identified the changeset that introduced the problem.