fix: handle nothing passed as u0 to ODEProblem

src/systems/diffeqs/abstractodesystem.jl

@@ -817,11 +817,12 @@ function process_DEProblem(constructor, sys::AbstractODESystem, u0map, parammap;
          ModelingToolkit.get_tearing_state(sys) !== nothing) ||
         !isempty(initialization_equations(sys))) && t !== nothing
         if eltype(u0map) <: Number
-            u0map = unknowns(sys) .=> u0map
+            u0map = unknowns(sys) .=> vec(u0map)
         end
-        if isempty(u0map)
+        if u0map === nothing || isempty(u0map)
             u0map = Dict()
         end
+
         initializeprob = ModelingToolkit.InitializationProblem(
             sys, t, u0map, parammap; guesses, warn_initialize_determined,
             initialization_eqs, eval_expression, eval_module, fully_determined, check_units)

src/systems/discrete_system/discrete_system.jl

@@ -247,6 +247,13 @@ function process_DiscreteProblem(constructor, sys::DiscreteSystem, u0map, paramm
     dvs = unknowns(sys)
     ps = parameters(sys)
 
+    if eltype(u0map) <: Number
+        u0map = unknowns(sys) .=> vec(u0map)
+    end
+    if u0map === nothing || isempty(u0map)
+        u0map = Dict()
+    end
+
     trueu0map = Dict()
     for (k, v) in u0map
         k = unwrap(k)

test/discrete_system.jl

@@ -271,3 +271,11 @@ end
 k = ShiftIndex(t)
 @named sys = DiscreteSystem([x ~ x^2 + y^2, y ~ x(k - 1) + y(k - 1)], t)
 @test_throws ["algebraic equations", "not yet supported"] structural_simplify(sys)
+
+@testset "Passing `nothing` to `u0`" begin
+    @variables x(t) = 1
+    k = ShiftIndex()
+    @mtkbuild sys = DiscreteSystem([x(k) ~ x(k - 1) + 1], t)
+    prob = @test_nowarn DiscreteProblem(sys, nothing, (0.0, 1.0))
+    @test_nowarn solve(prob, FunctionMap())
+end

test/nonlinearsystem.jl

@@ -318,3 +318,10 @@ sys = structural_simplify(ns; conservative = true)
     sol = solve(prob, NewtonRaphson())
     @test sol[x] ≈ sol[y] ≈ sol[z] ≈ -3
 end
+
+@testset "Passing `nothing` to `u0`" begin
+    @variables x = 1
+    @mtkbuild sys = NonlinearSystem([0 ~ x^2 - x^3 + 3])
+    prob = @test_nowarn NonlinearProblem(sys, nothing)
+    @test_nowarn solve(prob)
+end

test/odesystem.jl

@@ -1387,3 +1387,10 @@ end
 
     @test obsfn(ones(2), 2ones(2), 3ones(4), 4.0) == 6ones(2)
 end
+
+@testset "Passing `nothing` to `u0`" begin
+    @variables x(t) = 1
+    @mtkbuild sys = ODESystem(D(x) ~ t, t)
+    prob = @test_nowarn ODEProblem(sys, nothing, (0.0, 1.0))
+    @test_nowarn solve(prob)
+end

test/optimizationsystem.jl

@@ -340,3 +340,10 @@ end
     prob.f.cons_h(H3, [1.0, 1.0], [1.0, 100.0])
     @test prob.f.cons_h([1.0, 1.0], [1.0, 100.0]) == H3
 end
+
+@testset "Passing `nothing` to `u0`" begin
+    @variables x = 1.0
+    @mtkbuild sys = OptimizationSystem((x - 3)^2, [x], [])
+    prob = @test_nowarn OptimizationProblem(sys, nothing)
+    @test_nowarn solve(prob, NelderMead())
+end

test/sdesystem.jl

@@ -776,3 +776,11 @@ end
     prob = SDEProblem(de, u0map, (0.0, 100.0), parammap)
     @test solve(prob, SOSRI()).retcode == ReturnCode.Success
 end
+
+@testset "Passing `nothing` to `u0`" begin
+    @variables x(t) = 1
+    @brownian b
+    @mtkbuild sys = System([D(x) ~ x + b], t)
+    prob = @test_nowarn SDEProblem(sys, nothing, (0.0, 1.0))
+    @test_nowarn solve(prob, ImplicitEM())
+end