Forgetting a variable in `PDESystem` results in infinite loop

[bertini97]

Describe the bug 🐞

Consider

using MethodOfLines, ModelingToolkit, DomainSets

@parameters t, v
@variables p(..) S(..)

∂ₜ = Differential(t)
∂ᵥ = Differential(v)

eq = [ S(t, v) ~ - p(t, v) - ∂ᵥ(p(t, v)),
       ∂ₜ(p(t, v)) ~ -∂ᵥ(S(t, v)) ]

bcs = [ p(0.0, v) ~ 1.0, p(t, 20.0) ~ 0.0, ∂ᵥ(p(t, -10.0)) ~ 0.0 ]
domains = [t ∈ Interval(0.0, 1.0), v ∈ Interval(-10.0, 20.0)]

@named sys = PDESystem(eq, bcs, domains, [t, v], [p(t, v) S(t, v)])
discretization = MOLFiniteDifference([v => 0.5], t)
prob = discretize(sys, discretization)

If one forgets to add S(t, v) and just has

@named sys = PDESystem(eq, bcs, domains, [t, v], [p(t, v)])

it will result in infinite printing of

┌ Warning: Expanding derivatives in term Differential(v)(S(t, v)).
└ @ MethodOfLines ~/.julia/packages/MethodOfLines/JDu9X/src/system_parsing/pde_system_transformation.jl:19

Expected behavior

If there is a way to catch and handle this, maybe reporting it to the user, it would be good to have. Otherwise, just close the issue. Thanks!

[ChrisRackauckas]

It would be good to better catch this, thanks for the issue!