plot_rxns doesn't work for many products

[zxdawn]

I have tried to use plot_rxns for the O3 product by python -m permm --gui test.mrg.nc.

Steps:
I selected nothing in the "Select Rectants" column and checked "AND", then chose O3 in the "Select Products" column.

Error info:

Traceback (most recent call last):
  File "/home/xin/miniconda3/envs/python37/lib/python3.8/tkinter/__init__.py", line 1883, in __call__
    return self.func(*args)
  File "/home/xin/miniconda3/envs/python37/lib/python3.8/site-packages/permm-1.0-py3.8.egg/permm/guis/SimpleTk.py", line 87, in execute
    getattr(self.mech, method)(**thiskwds)
  File "/home/xin/miniconda3/envs/python37/lib/python3.8/site-packages/permm-1.0-py3.8.egg/permm/core/Mechanism.py", line 597, in plot_rxns
    reactions = [(rxn.sum()[plot_spc], rxn) for rxn in reactions]
  File "/home/xin/miniconda3/envs/python37/lib/python3.8/site-packages/permm-1.0-py3.8.egg/permm/core/Mechanism.py", line 597, in <listcomp>
    reactions = [(rxn.sum()[plot_spc], rxn) for rxn in reactions]
AttributeError: 'str' object has no attribute 'sum'

I printed the reactions before the rxn.sum():

permm/src/permm/core/Mechanism.py

Line 596 in 5f2b03a

reactions = [(rxn.sum()[plot_spc], rxn) for rxn in reactions]

Here's the output:

['RXN_02', 'RXN_92', 'RXN_108']

This issue exists for many products (O, O3, H2O2, NO, etc).