Catch empty controls in optimization

Trying to run `optimize` when there were no controls in the problem
would produce a very non-obvious MethodError. Since this is an easy
mistake to make, we can catch this situation with a proper error
message.

src/workspace.jl

@@ -68,6 +68,9 @@ function KrotovWrk(problem::QuantumControlBase.ControlProblem; verbose=false)
     objectives = [obj for obj in problem.objectives]
     adjoint_objectives = [adjoint(obj) for obj in problem.objectives]
     controls = get_controls(objectives)
+    if length(controls) == 0
+        error("no controls in objectives: cannot optimize")
+    end
     control_derivs = [get_control_derivs(obj.generator, controls) for obj in objectives]
     tlist = problem.tlist
     kwargs = Dict(problem.kwargs)  # creates a shallow copy; ok to modify

test/runtests.jl

@@ -16,6 +16,11 @@ include(joinpath(@__DIR__, "download_dumps.jl"))
         include("test_pulse_optimization.jl")
     end
 
+    print("\n* Emptry Optimization (test_empty_optimization.jl)")
+    @time @safetestset "Empty Optimization" begin
+        include("test_empty_optimization.jl")
+    end
+
     print("\n* Example 1 (examples/simple_state_to_state.jl):")
     @time @safetestset "Example 1" begin
         include(joinpath("examples", "simple_state_to_state.jl"))

test/test_empty_optimization.jl

@@ -0,0 +1,33 @@
+using Test
+using StableRNGs
+using QuantumControl: hamiltonian, optimize, ControlProblem, Objective
+using QuantumControl.Controls: get_controls
+using QuantumControlTestUtils.RandomObjects: random_matrix, random_state_vector
+
+@testset "empty optimization" begin
+
+    # Test that trying to run an optimization without any controls produces a
+    # meaningful error message
+
+    rng = StableRNG(2264511904)
+
+    N = 10
+    H = random_matrix(N; rng)
+    objectives = [
+        Objective(;
+            initial_state=random_state_vector(N; rng),
+            generator=H,
+            target_state=random_state_vector(N; rng)
+        )
+    ]
+
+    @test length(get_controls(objectives)) == 0
+
+    tlist = collect(range(0; length=1001, step=1.0))
+
+    problem = ControlProblem(; objectives, tlist, pulse_options=Dict())
+
+    msg = "no controls in objectives: cannot optimize"
+    @test_throws ErrorException(msg) optimize(problem; method=:krotov)
+
+end