`plot_statistics` does not work when `model_topology.topological_boundaries` is set to `state`

Version Checks (indicate both or one)

• This bug exists on the master branch of PyPSA-USA.

• This bug exists on the develop branch of PyPSA-USA.

The Issue

If I set the topological_boundaries to be state I get the error message below on an solved network with an optimal solution

Error in rule plot_statistics:
    jobid: 24
    input: results/imports_test_state/western/networks/elec_s50_c12_ec_lv1.0_3h_E.nc, resources/imports_test_state/western/Geospatial/regions_onshore_s50_12.geojson, resources/imports_test_state/western/Geospatial/regions_offshore_s50_12.geojson
    output: results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/emissions/emissions_area.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/emissions/emissions_accumulated.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/emissions/emissions_accumulated_tech.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/emissions/bar_regional_emissions.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/emissions/bar_emissions.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/production/production_area.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/system/production_bar.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/system/capacity_additions_bar.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/system/bar_regional_capacity_additions.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/system/bar_regional_production.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/system/global_constraint_shadow_prices.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/system/generator_data_panel.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/system/fuel_costs.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/system/region_lmps.pdf, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/statistics/statistics.csv, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/statistics/statistics_dissaggregated.csv, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/statistics/generators.csv, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/statistics/storage_units.csv, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/statistics/links.csv, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/statistics/buses.csv, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/statistics/lines.csv, results/imports_test_state/western/figures/s50_cluster_12/lv1.0_3h_E/statistics/stores.csv
    log: logs/plot_figures/western_50_12_lv1.0_3h_E.log (check log file(s) for error details)

Steps To Reproduce

# docs :
scenario:
  interconnect: [western] #"usa|texas|western|eastern"
  clusters: [12]
  simpl: [50]
  opts: [3h]
  ll: [v1.0]
  sector: "" # G
  planning_horizons: [2030]    #(2018-2023, 2030, 2040, 2050)
foresight:  'perfect' # myopic, perfect

model_topology:
  transmission_network: 'reeds' # [reeds, tamu]
  topological_boundaries: 'state' # [county, reeds_zone, state]
  interface_transmission_limits: false
  include: # mixed zone types not supported
    # reeds_zone: []
    # reeds_state: ["CA", "OR"]
    # reeds_ba: []
  aggregate: # eligible keys: [reeds_zone, trans_reg]
    # trans_grp: []
    # reeds_zone: []
  # trim: 
  #   zone: ['CA']

Error Message

Traceback (most recent call last):
  File "/home/trevor/master/pypsa-usa/workflow/.snakemake/scripts/tmp88kr109_.plot_statistics.py", line 989, in <module>
    plot_regional_capacity_additions_bar(
  File "/home/trevor/master/pypsa-usa/workflow/.snakemake/scripts/tmp88kr109_.plot_statistics.py", line 404, in plot_regional_capacity_additions_bar
    data = get_statistics(n, "Optimal Capacity")
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/trevor/master/pypsa-usa/workflow/.snakemake/scripts/tmp88kr109_.plot_statistics.py", line 297, in get_statistics
    gens_reg = gens.map(n.generators.bus.map(n.buses.nerc_reg)).to_series()
                                             ^^^^^^^^^^^^^^^^
  File "/home/trevor/miniforge3/envs/pypsa-usa/lib/python3.11/site-packages/pandas/core/generic.py", line 6299, in __getattr__
    return object.__getattribute__(self, name)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'DataFrame' object has no attribute 'nerc_reg'

Anything else?

No response

Start with the plot_statistics entry point and the generated workflow/.snakemake/scripts/tmp88kr109_.plot_statistics.py traceback, especially get_statistics and plot_regional_capacity_additions_bar. Reproduce the state-boundary configuration and inspect the referenced network bus data. Done means plot_statistics completes for topological_boundaries: state and produces the listed statistics and figure outputs without the nerc_reg error.