Printing the result of a run gives an error when flow_name is None #536

from sklearn import ensemble
from openml import tasks, flows, runs
import openml

task = tasks.get_task(3954)
clf = ensemble.RandomForestClassifier()
flow = flows.sklearn_to_flow(clf)
run = runs.run_flow_on_task(task, flow)
print(run)   #error

In [87]: run.flow_name is None
Out[87]: True

In [88]: print(run)
Out[88]: 
[run id: None, task id: 3954, flow id: 8656, flow name: None]

In [74]: print(run)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-74-7667ec7d87d4> in <module>()
----> 1 print(run)

/usr/local/lib/python3.6/site-packages/openml/runs/run.py in __str__(self)
     57     def __str__(self):
     58         flow_name = self.flow_name
---> 59         if len(flow_name) > 26:
     60             # long enough to show sklearn.pipeline.Pipeline
     61             flow_name = flow_name[:26] + "..."

TypeError: object of type 'NoneType' has no len()