ValueError: No value for uninitialized NumericValue object

# :beetle:

- [x] I have checked that this issue has not already been reported.

### Bug summary
When running a Switch model using the code currently in the `mvp` branch, the solver finds an optimal solution, and writes outputs to a pickle file, but then throws a ValueError.

### Code for reproduction
Preparation:
```python
import os

import pandas as pd
from powersimdata import Scenario

from switchwrapper.prepare import prepare_inputs

scenario = Scenario(599)
grid = scenario.get_grid()
profiles = {
"demand": scenario.get_demand(),
"hydro": scenario.get_hydro(),
"solar": scenario.get_solar(),
"wind": scenario.get_wind(),
}
timepoints = pd.read_csv("timepoints_input_v2.csv", index_col=0)
timestamps_to_timepoints = pd.read_csv("slicing_recovery.csv", index_col=0).squeeze()
prepare_inputs(grid, profiles, timepoints, timestamps_to_timepoints, os.path.join("prepared", "inputs"))
```
Launching:
```shell
solve --verbose --solver gurobi --suffixes dual
```

### Actual outcome
The output produced by the above code, which may be a screenshot, console output, etc.
```shell
solve --verbose --solver gurobi --suffixes dual

=======================================================================
Switch 2.0.6, http://switch-model.org
=======================================================================
Arguments:
logs_dir='logs', inputs_dir='inputs', suffixes=['dual'], solver='gurobi', solver_manager='serial', outputs_dir='outputs', verbose=True
Modules:
switch_model, switch_model.timescales, switch_model.financials, switch_model.balancing.load_zones, switch_model.energy_sources.properties, switch_model.generators.core.build, switch_model.generators.core.dispatch, switch_model.generators.core.no_commit, switch_model.energy_sources.fuel_costs.simple, switch_model.transmission.local_td, switch_model.transmission.transport.build, switch_model.transmission.transport.dispatch, switch_model.reporting, switch_model.solve
=======================================================================

Model created in 0.24 s.
Loading inputs...
Data read in 3.67 s.

Instance created from data in 161.02 s.

Total time spent constructing model: 164.69 s.

Solving model...
Solved model. Total time spent in solver: 140.247918 s.

Optimization termination condition was optimal.
Solver message: Model was solved to optimality (subject to tolerances), and an optimal solution is available.

Saved results in 22.09 s.
Executing post solve functions...
ERROR: evaluating object as numeric value: BuildTx[88607ac,2030]
(object: )
No value for uninitialized NumericValue object BuildTx[88607ac,2030]
ERROR: evaluating object as numeric value: TxCapacityNameplate[88607ac,2030]
(object: )
No value for uninitialized NumericValue object BuildTx[88607ac,2030]
Traceback (most recent call last):
File "/home/dolsen/.local/bin/switch", line 8, in
sys.exit(main())
File "/home/dolsen/.local/lib/python3.8/site-packages/switch_model/main.py", line 39, in main
main()
File "/home/dolsen/.local/lib/python3.8/site-packages/switch_model/solve.py", line 183, in main
instance.post_solve()
File "/home/dolsen/.local/lib/python3.8/site-packages/switch_model/utilities.py", line 252, in post_solve
module.post_solve(instance, outputs_dir)
File "/home/dolsen/.local/lib/python3.8/site-packages/switch_model/transmission/transport/build.py", line 336, in post_solve
normalized_dat = [
File "/home/dolsen/.local/lib/python3.8/site-packages/switch_model/transmission/transport/build.py", line 346, in
"TxCapacityNameplate": value(mod.TxCapacityNameplate[tx,p]),
File "/home/dolsen/.local/lib/python3.8/site-packages/pyomo/core/expr/numvalue.py", line 226, in value
tmp = obj(exception=True)
File "/home/dolsen/.local/lib/python3.8/site-packages/pyomo/core/base/expression.py", line 54, in __call__
return self.expr(exception=exception)
File "/home/dolsen/.local/lib/python3.8/site-packages/pyomo/core/expr/numeric_expr.py", line 223, in __call__
return evaluate_expression(self, exception)
File "/home/dolsen/.local/lib/python3.8/site-packages/pyomo/core/expr/visitor.py", line 974, in evaluate_expression
return visitor.dfs_postorder_stack(exp)
File "/home/dolsen/.local/lib/python3.8/site-packages/pyomo/core/expr/visitor.py", line 519, in dfs_postorder_stack
flag, value = self.visiting_potential_leaf(_sub)
File "/home/dolsen/.local/lib/python3.8/site-packages/pyomo/core/expr/visitor.py", line 894, in visiting_potential_leaf
return True, value(node)
File "/home/dolsen/.local/lib/python3.8/site-packages/pyomo/core/expr/numvalue.py", line 228, in value
raise ValueError(
ValueError: No value for uninitialized NumericValue object BuildTx[88607ac,2030]
```

### Expected outcome
No error.

### Environment
Please specify your platform and versions of the relevant libraries you are using:
* Operating system: Linux
* SwitchWrapper revision (run `git rev-parse origin/HEAD`): `b162f259b1d678226246964f2980f695c2d21e19`
* Python version: 3.8.5
* Other libraries:
* Pyomo 5.6.8
* switch-model 2.0.6