Model.results() error in oemof 3.1

Hello,
I made a Modell in oemof which worked perfectly fine in oemof 2.4 and now I wanted to test it in oemof 3.1 and got this error:

Traceback (most recent call last):
File “/usr/lib/python3.7/code.py”, line 90, in runcode
exec(code, self.locals)
File “”, line 1, in
File “/snap/pycharm-community/132/helpers/pydev/_pydev_bundle/pydev_umd.py”, line 197, in runfile
pydev_imports.execfile(filename, global_vars, local_vars) # execute the script
File “/snap/pycharm-community/132/helpers/pydev/_pydev_imps/_pydev_execfile.py”, line 18, in execfile
exec(compile(contents+"\n", file, ‘exec’), glob, loc)
File “/home/philipp/Schreibtisch/oemof_new/oemof/solph/test_systems/1_system.py”, line 76, in
model.results()
File “/home/philipp/Schreibtisch/oemof_new/oemof/solph/models.py”, line 146, in results
result = processing.results(self)
File “/home/philipp/Schreibtisch/oemof_new/oemof/outputlib/processing.py”, line 111, in results
df = create_dataframe(om)
File “/home/philipp/Schreibtisch/oemof_new/oemof/outputlib/processing.py”, line 89, in create_dataframe
df[‘timestep’] = df[‘oemof_tuple’].map(get_timestep)
File “/home/philipp/.local/lib/python3.7/site-packages/pandas/core/series.py”, line 2998, in map
arg, na_action=na_action)
File “/home/philipp/.local/lib/python3.7/site-packages/pandas/core/base.py”, line 1004, in _map_values
new_values = map_f(values, mapper)
File “pandas/_libs/src/inference.pyx”, line 1472, in pandas._libs.lib.map_infer
File “/home/philipp/Schreibtisch/oemof_new/oemof/outputlib/processing.py”, line 45, in get_timestep
if all(issubclass(type(n), Node) for n in x):
TypeError: ‘NoneType’ object is not iterable

I am even able to solve the model, but when I use:
model.results()
I get an error… Has anyone an idea what the problem could be?

Best, Philipp