Add error checking

Author: YingboMa

src/systems/abstractsystem.jl

@@ -746,6 +746,7 @@ end
 # to an `ODESystem` to connect systems, and we later can reply on
 # `structural_simplify` to convert `ODESystem`s to `NonlinearSystem`s.
 function toodesystem(sys, t; name=nameof(sys))
+    isempty(observed(sys)) || throw(ArgumentError("`toodesystem` cannot handle reduced model (i.e. observed(sys) is non-empty)."))
     t = value(t)
     varmap = Dict()
     sts = states(sys)

test/nonlinearsystem.jl

@@ -106,3 +106,4 @@ sys = structural_simplify(sys)
 prob = ODEProblem(sys, [subsys.x => 1, subsys.z => 2.0], (0, 1.0), [subsys.σ=>1,subsys.ρ=>2,subsys.β=>3])
 sol = solve(prob, Rodas5())
 @test sol[subsys.x] + sol[subsys.y] - sol[subsys.z] ≈ sol[subsys.u]
+@test_throws ArgumentError toodesystem(sys, t)