fix: fix step! without dt for NullODEIntegrator

Author: AayushSabharwal

src/solve.jl

@@ -693,7 +693,7 @@ function step!(integ::NullODEIntegrator, dt = nothing, stop_at_tdt = false)
     if !isnothing(dt)
         integ.t += dt
     else
-        integ.t = integ.sol[end]
+        integ.t = integ.sol.t[end]
     end
     return nothing
 end

test/downstream/null_de.jl

@@ -95,3 +95,10 @@ end
     @test_nowarn SciMLBase.u_modified!(integ, Float64[])
     @test SciMLBase.successful_retcode(SciMLBase.check_error(integ))
 end
+
+@testset "`step!` without dt" begin
+    prob = ODEProblem(Returns(nothing), nothing, (0.0, 1.0))
+    integ = init(prob, Tsit5())
+    @test_nowarn step!(integ)
+    @test integ.t ≈ 1.0
+end