using `ContinuousBoundaryFunction` causes `FieldTimeSeries` read with `OnDisk()` to fail

[xkykai]

Below is an MWE:

using Oceananigans

grid = RectilinearGrid(CPU(), size = (4, 4), extent = (1, 1), topology = (Bounded, Flat, Bounded))

b_bc(y, z, t) = tanh(t)

b_bcs = FieldBoundaryConditions(west = FluxBoundaryCondition(b_bc))

model = NonhydrostaticModel(; grid,
                              tracers = :b,
                              buoyancy = BuoyancyTracer(),
                              boundary_conditions = (; b = b_bcs))

simulation = Simulation(model; Δt = 1, stop_time = 2)

simulation.output_writers[:jld2] = JLD2Writer(model, (; b = model.tracers.b);
                                              filename = joinpath("./instantaneous_fields.jld2"),
                                              schedule = IterationInterval(1),
                                              with_halos = true,
                                              overwrite_existing = true)

run!(simulation)

b_data = FieldTimeSeries("./instantaneous_fields.jld2", "b", backend=OnDisk())
# b_data = FieldTimeSeries("./instantaneous_fields.jld2", "b")

b_data[1]
interior(b_data[1])

doing b_data[1] gives

julia> b_data[1]
ERROR: MethodError: objects of type Nothing are not callable
The object of type `Nothing` exists, but no method is defined for this combination of argument types when trying to treat it as a callable object.
Stacktrace:
 [1] fill_halo_event!
   @ c:\Users\xinle\Dropbox\MIT\main_oceananigans\Oceananigans.jl\src\BoundaryConditions\fill_halo_regions.jl:40 [inlined]
 [2] fill_halo_regions!(::OffsetArrays.OffsetArray{…}, ::FieldBoundaryConditions{…}, ::Tuple{…}, ::Tuple{…}, ::RectilinearGrid{…}; kwargs::@Kwargs{})
   @ Oceananigans.BoundaryConditions c:\Users\xinle\Dropbox\MIT\main_oceananigans\Oceananigans.jl\src\BoundaryConditions\fill_halo_regions.jl:32
 [3] fill_halo_regions!
   @ c:\Users\xinle\Dropbox\MIT\main_oceananigans\Oceananigans.jl\src\BoundaryConditions\fill_halo_regions.jl:25 [inlined]
 [4] #fill_halo_regions!#62
   @ c:\Users\xinle\Dropbox\MIT\main_oceananigans\Oceananigans.jl\src\Fields\field.jl:852 [inlined]
 [5] fill_halo_regions!(::Field{…})
   @ Oceananigans.Fields c:\Users\xinle\Dropbox\MIT\main_oceananigans\Oceananigans.jl\src\Fields\field.jl:838
 [6] getindex(fts::FieldTimeSeries{…}, n::Int64)
   @ Oceananigans.OutputReaders c:\Users\xinle\Dropbox\MIT\main_oceananigans\Oceananigans.jl\src\OutputReaders\field_time_series_indexing.jl:158
 [7] top-level scope
   @ REPL[38]:1
Some type information was truncated. Use `show(err)` to see complete types.

which is also similar to when we do interior(b_data[1]). However if we do

b_data = FieldTimeSeries("./instantaneous_fields.jld2", "b")

then b_data[1] gives

Error showing value of type Field{Center, Center, Center, Nothing, RectilinearGrid{Float64, Bounded, Flat, Bounded, Oceananigans.Grids.StaticVerticalDiscretization{OffsetArrays.OffsetVector{Float64, StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}}, OffsetArrays.OffsetVector{Float64, StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}}, Float64, Float64}, Float64, Float64, OffsetArrays.OffsetVector{Float64, StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64}}, Nothing, CPU}, Tuple{Colon, Colon, Colon}, OffsetArrays.OffsetArray{Float64, 3, SubArray{Float64, 3, Array{Float64, 4}, Tuple{Base.Slice{Base.OneTo{Int64}}, Base.Slice{Base.OneTo{Int64}}, Base.Slice{Base.OneTo{Int64}}, Int64}, true}}, Float64, FieldBoundaryConditions{Missing, Oceananigans.BoundaryConditions.NoFluxBoundaryCondition, Nothing, Nothing, Oceananigans.BoundaryConditions.NoFluxBoundaryCondition, Oceananigans.BoundaryConditions.NoFluxBoundaryCondition, Nothing, @NamedTuple{bottom_and_top::KernelAbstractions.Kernel{KernelAbstractions.CPU, KernelAbstractions.NDIteration.StaticSize{(4, 1)}, KernelAbstractions.NDIteration.StaticSize{(4, 1)}, typeof(Oceananigans.BoundaryConditions.cpu__fill_bottom_and_top_halo!)}, south_and_north::Nothing, west_and_east::Nothing}, @NamedTuple{bottom_and_top::Tuple{Oceananigans.BoundaryConditions.NoFluxBoundaryCondition, Oceananigans.BoundaryConditions.NoFluxBoundaryCondition}, south_and_north::Tuple{Nothing, Nothing}, west_and_east::Tuple{Missing, Oceananigans.BoundaryConditions.NoFluxBoundaryCondition}}}, Oceananigans.Fields.FixedTime{Float64}, Nothing}:
ERROR: MethodError: no method matching bc_str(::Missing)
The function `bc_str` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  bc_str(::BoundaryCondition{Oceananigans.BoundaryConditions.Open, Nothing})
   @ Oceananigans c:\Users\xinle\Dropbox\MIT\main_oceananigans\Oceananigans.jl\src\BoundaryConditions\show_boundary_conditions.jl:13
  bc_str(::Oceananigans.BoundaryConditions.NoFluxBoundaryCondition)
   @ Oceananigans c:\Users\xinle\Dropbox\MIT\main_oceananigans\Oceananigans.jl\src\BoundaryConditions\show_boundary_conditions.jl:12
  bc_str(::Nothing)
   @ Oceananigans c:\Users\xinle\Dropbox\MIT\main_oceananigans\Oceananigans.jl\src\BoundaryConditions\show_boundary_conditions.jl:17
  ...

Stacktrace:
  [1] show(io::IOContext{Base.TTY}, field::Field{Center, Center, Center, Nothing, RectilinearGrid{…}, Tuple{…}, OffsetArrays.OffsetArray{…}, Float64, FieldBoundaryConditions{…}, Oceananigans.Fields.FixedTime{…}, Nothing})
    @ Oceananigans.Fields c:\Users\xinle\Dropbox\MIT\main_oceananigans\Oceananigans.jl\src\Fields\show_fields.jl:43
  [2] show(io::IOContext{Base.TTY}, ::MIME{Symbol("text/plain")}, f::Field{Center, Center, Center, Nothing, RectilinearGrid{…}, Tuple{…}, OffsetArrays.OffsetArray{…}, Float64, FieldBoundaryConditions{…}, Oceananigans.Fields.FixedTime{…}, Nothing})
    @ Oceananigans.Fields c:\Users\xinle\Dropbox\MIT\main_oceananigans\Oceananigans.jl\src\Fields\show_fields.jl:74
  [3] (::REPL.var"#68#69"{REPL.REPLDisplay{REPL.LineEditREPL}, MIME{Symbol("text/plain")}, Base.RefValue{Any}})(io::Any)
    @ REPL C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\REPL.jl:367
  [4] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
    @ REPL C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\REPL.jl:661
  [5] display(d::REPL.REPLDisplay, mime::MIME{Symbol("text/plain")}, x::Any)
    @ REPL C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\REPL.jl:353
  [6] display(d::REPL.REPLDisplay, x::Any)
    @ REPL C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\REPL.jl:372
  [7] display(x::Any)
    @ Base.Multimedia .\multimedia.jl:340
  [8] #invokelatest#2
    @ .\essentials.jl:1055 [inlined]
  [9] invokelatest
    @ .\essentials.jl:1052 [inlined]
 [10] print_response(errio::IO, response::Any, show_value::Bool, have_color::Bool, specialdisplay::Union{Nothing, AbstractDisplay})
    @ REPL C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\REPL.jl:409
 [11] (::REPL.var"#70#71"{REPL.LineEditREPL, Pair{Any, Bool}, Bool, Bool})(io::Any)
    @ REPL C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\REPL.jl:378
 [12] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
    @ REPL C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\REPL.jl:661
 [13] print_response(repl::REPL.AbstractREPL, response::Any, show_value::Bool, have_color::Bool)
    @ REPL C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\REPL.jl:376
 [14] (::REPL.var"#do_respond#96"{Bool, Bool, REPL.var"#112#130"{REPL.LineEditREPL, REPL.REPLHistoryProvider}, REPL.LineEditREPL, REPL.LineEdit.Prompt})(s::REPL.LineEdit.MIState, buf::Any, ok::Bool)
    @ REPL C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\REPL.jl:1003
 [15] (::VSCodeServer.var"#103#106"{REPL.var"#do_respond#96"{Bool, Bool, REPL.var"#112#130"{REPL.LineEditREPL, REPL.REPLHistoryProvider}, REPL.LineEditREPL, REPL.LineEdit.Prompt}})(mi::REPL.LineEdit.MIState, buf::IOBuffer, ok::Bool)
    @ VSCodeServer c:\Users\xinle\.vscode\extensions\julialang.language-julia-1.158.2\scripts\packages\VSCodeServer\src\repl.jl:122
 [16] #invokelatest#2
    @ .\essentials.jl:1055 [inlined]
 [17] invokelatest
    @ .\essentials.jl:1052 [inlined]
 [18] run_interface(terminal::REPL.Terminals.TextTerminal, m::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
    @ REPL.LineEdit C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\LineEdit.jl:2755
 [19] run_frontend(repl::REPL.LineEditREPL, backend::REPL.REPLBackendRef)
    @ REPL C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\REPL.jl:1474
 [20] (::REPL.var"#75#81"{REPL.LineEditREPL, REPL.REPLBackendRef})()
    @ REPL C:\Users\xinle\AppData\Local\Programs\Julia-1.11.4\share\julia\stdlib\v1.11\REPL\src\REPL.jl:480
Some type information was truncated. Use `show(err)` to see complete types.

while interior(b_data[1]) works.