Solution interface does not work when using init() and step!()

[aplund]

It seems that one cannot use the solution using init() and step!() from the integrator interface.

using ModelingToolkit, MethodOfLines, OrdinaryDiffEq, DomainSets


@variables x y t
@variables u(..) v(..)
Dt = Differential(t)
Dx = Differential(x)
Dy = Differential(y)
Dxx = Differential(x)^2
Dyy = Differential(y)^2

∇²(u) = Dxx(u) + Dyy(u)

brusselator_f(x, y, t) = (((x-0.3)^2 + (y-0.6)^2) <= 0.1^2) * (t >= 1.1) * 5.

x_min = y_min = t_min = 0.0
x_max = y_max = 1.0
t_max = 11.5

α = 10.

u0(x,y,t) = 22(y*(1-y))^(3/2)
v0(x,y,t) = 27(x*(1-x))^(3/2)

eq = [Dt(u(x,y,t)) ~ 1. + v(x,y,t)*u(x,y,t)^2 - 4.4*u(x,y,t) + α*∇²(u(x,y,t)) + brusselator_f(x, y, t),
       Dt(v(x,y,t)) ~ 3.4*u(x,y,t) - v(x,y,t)*u(x,y,t)^2 + α*∇²(v(x,y,t))]

domains = [x ∈ Interval(x_min, x_max),
              y ∈ Interval(y_min, y_max),
              t ∈ Interval(t_min, t_max)]

# Periodic BCs
bcs = [u(x,y,0) ~ u0(x,y,0),
       u(0,y,t) ~ u(1,y,t),
       u(x,0,t) ~ u(x,1,t),

       v(x,y,0) ~ v0(x,y,0),
       v(0,y,t) ~ v(1,y,t),
       v(x,0,t) ~ v(x,1,t)]

@named pdesys = PDESystem(eq,bcs,domains,[x,y,t],[u(x,y,t),v(x,y,t)])

N = 32

order = 2 # This may be increased to improve accuracy of some schemes

# Integers for x and y are interpreted as number of points. Use a Float to directtly specify stepsizes dx and dy.
discretization = MOLFiniteDifference([x=>N, y=>N], t, approx_order=order)

# Convert the PDE problem into an ODE problem
println("Discretization:")
@time prob = discretize(pdesys,discretization)

state = init(prob, TRBDF2())
step!(state, 0.1)

discrete_x = state.sol[x]
discrete_y = state.sol[y]
discrete_t = state.sol[t]

solu = state.sol[u(x, y, t)]
solv = state.sol[v(x, y, t)]

The error given is

ERROR: LoadError: ArgumentError: x is neither an observed nor a state variable.

with an extraordinarily long stack trace starting at build_explicit_observed_function.