fix: handle nothing passed as u0 to ODEProblem

Author: AayushSabharwal

src/systems/diffeqs/abstractodesystem.jl

@@ -811,17 +811,18 @@ function process_DEProblem(constructor, sys::AbstractODESystem, u0map, parammap;
         end
     end
 
+    if eltype(u0map) <: Number
+        u0map = unknowns(sys) .=> u0map
+    end
+    if u0map === nothing || isempty(u0map)
+        u0map = Dict()
+    end
+
     # ModelingToolkit.get_tearing_state(sys) !== nothing => Requires structural_simplify first
     if sys isa ODESystem && build_initializeprob &&
        (((implicit_dae || !isempty(missingvars) || !isempty(setobserved)) &&
          ModelingToolkit.get_tearing_state(sys) !== nothing) ||
         !isempty(initialization_equations(sys))) && t !== nothing
-        if eltype(u0map) <: Number
-            u0map = unknowns(sys) .=> u0map
-        end
-        if 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)

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 = ODEProblem(D(x) ~ t, t)
+    prob = @test_nowarn ODEProblem(sys, nothing, (0.0, 1.0))
+    @test_nowarn solve(prob)
+end