fix MatrixOperator mass_matrix

Author: oscardssmith

lib/OrdinaryDiffEqDifferentiation/src/derivative_utils.jl

@@ -718,6 +718,9 @@ function calc_W!(W, integrator, nlsolver::Union{Nothing, AbstractNLSolver}, cach
             if W.J isa MatrixOperator
                 J = J.A
             end
+            if mass_matrix isa MatrixOperator
+                mass_matrix = mass_matrix.A
+            end
             if new_W && !isdae
                 jacobian2W!(W._concrete_form, mass_matrix, dtgamma, J, W_transform)
             end