Merge pull request #592 from djarecka/fix/out_error

adding an Attribute Error to the exception for get_value

pydra/engine/core.py

@@ -1214,7 +1214,7 @@ def _collect_outputs(self):
             try:
                 val_out = val.get_value(self)
                 output_wf[name] = val_out
-            except ValueError:
+            except (ValueError, AttributeError):
                 output_wf[name] = None
                 # checking if the tasks has predecessors that raises error
                 if isinstance(getattr(self, val.name)._errored, list):