`flows()` doesn't work if a scenario is loaded from a .dflx file

[jschmidl]

found this issue when trying to create a graph from a scenario:

When running sc.plot_nodes(filename=graph_path) that i created and computed from a xlsx file it works fine. But when i dump the scenario and then load it again, the same function does not work anymore:

sc.dump(dump_path) sc1 = tools.files.restore_scenario(dump_path) sc1.plot_nodes(filename=graphPath)

will yield an error in the flows() function of oemof/networks/energy_systems.py.

It looks like somehow the scenario gets corrupted during the dump/load process... Maybe it's a pickle issue?

System: Windows 10
python-version: 3.8
deflex-version: "0.3.1dev0"
oemof-version: '0.3.2'