roiRectangles: 'NoneType' object has no attribute 'encode'

[will-moore]

Seen on demo server:

Traceback (most recent call last):
  File "/opt/omero/web/venv3/lib64/python3.6/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/opt/omero/web/venv3/lib64/python3.6/site-packages/django/core/handlers/base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/opt/omero/web/venv3/lib64/python3.6/site-packages/omeroweb/decorators.py", line 544, in wrapped
    retval = f(request, *args, **kwargs)
  File "/opt/omero/web/venv3/lib64/python3.6/site-packages/omero_figure/views.py", line 602, in roi_rectangles
    json_data.append(encoder.encode(roi))
  File "/opt/omero/web/venv3/lib64/python3.6/site-packages/omero_marshal/encode/encoders/roi.py", line 29, in encode
    shapes.append(shape_encoder.encode(shape))

Exception Type: AttributeError at /figure/roiRectangles/247940/
Exception Value: 'NoneType' object has no attribute 'encode'