Bug related to None value for "dest_sim.next_step" in particular connection structure

Dear,

Using mosaik 3.0.0, I've end up with the following error message

  File "C:\Users\DEK328\Anaconda3\envs\cosim\lib\site-packages\mosaik\scheduler.py", line 93, in sim_process
    notify_dependencies(world, sim)
  File "C:\Users\DEK328\Anaconda3\envs\cosim\lib\site-packages\mosaik\scheduler.py", line 525, in notify_dependencies
    elif sim.output_time < dest_sim.next_step:
TypeError: '<' not supported between instances of 'int' and 'NoneType'

I've reduce my code base to the minimum structure leading to this error. The structure is mostly as follow:

csv -----SIGNAL_CSV--|
                     |--> B ---SIGNAL_B----> collector
A   -----SIGNAL_CSV--|

One particularity is that the ModelB simulator has "SIGNAL_CSV" in its trigger list. Removing that remove the bug.

Here is a minimal code allowing to reproduce the bug examples.zip

Kind regards