Baron writer refuses to solve a deactivated block

[emma58]

Summary

If you pass a deactivated Block to baron, then it will die on an UnboundedLocalError. This is inconsistent with other solvers.

Steps to reproduce the issue

from pyomo.environ import *
m = ConcreteModel()
m.b = Block()
m.b.x = Var()
m.b.c = Constraint(expr=m.b.x >= 9)
m.b.o = Objective(expr=m.b.x)
m.b.deactivate()
SolverFactory('baron').solve(m.b, tee=True)

Error Message

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/esjohn/src/pyomo/pyomo/opt/base/solvers.py", line 598, in solve
    self._presolve(*args, **kwds)
  File "/home/esjohn/src/pyomo/pyomo/opt/solver/shellcmd.py", line 223, in _presolve
    OptSolver._presolve(self, *args, **kwds)
  File "/home/esjohn/src/pyomo/pyomo/opt/base/solvers.py", line 704, in _presolve
    self._convert_problem(
  File "/home/esjohn/src/pyomo/pyomo/solvers/plugins/solvers/BARON.py", line 273, in _convert_problem
    return OptSolver._convert_problem(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/esjohn/src/pyomo/pyomo/opt/base/solvers.py", line 756, in _convert_problem
    return convert_problem(
           ^^^^^^^^^^^^^^^^
  File "/home/esjohn/src/pyomo/pyomo/opt/base/convert.py", line 97, in convert_problem
    problem_files, symbol_map = converter.apply(*tmp, **tmpkw)
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/esjohn/src/pyomo/pyomo/solvers/plugins/converter/model.py", line 126, in apply
    (problem_filename, symbol_map_id) = instance.write(
                                        ^^^^^^^^^^^^^^^
  File "/home/esjohn/src/pyomo/pyomo/core/base/block.py", line 1927, in write
    (filename, smap) = problem_writer(self, filename, solver_capability, io_options)
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/esjohn/src/pyomo/pyomo/repn/plugins/baron_writer.py", line 517, in __call__
    symbol_map = self._write_bar_file(
                 ^^^^^^^^^^^^^^^^^^^^^
  File "/home/esjohn/src/pyomo/pyomo/repn/plugins/baron_writer.py", line 661, in _write_bar_file
    self._write_equations_section(
  File "/home/esjohn/src/pyomo/pyomo/repn/plugins/baron_writer.py", line 498, in _write_equations_section
    output_file.write(obj_string + ";\n\n")
                      ^^^^^^^^^^
UnboundLocalError: cannot access local variable 'obj_string' where it is not associated with a value

Information on your system

Pyomo version: main
Python version: 3.11
Operating system: linux
How Pyomo was installed (PyPI, conda, source): source
Solver (if applicable): baron

Additional information

Replacing baron with gurobi, the model solves fine.