Changing aggregator

[yydoe08]

Describe the bug

Changing Aggregator in DirectedGraphSAGE gives error below

Observed behavior

if aggregator is None:
870 self._aggregator = MeanAggregator
--> 871 elif issubclass(aggregator, Layer):
872 self._aggregator = aggregator
873 else:

TypeError: issubclass() arg 1 must be a class

The error happens when changing the aggregator. I think line 871 should check the instance of aggregator and Layer or change the code to below?
issubclass(type(aggregator), Layer)

[lunarrecluse]

Have you imported MaxPoolingAggregator?

from stellargraph.layer import MaxPoolingAggregator