Update reactionsystem_conversions.jl

Author: TorkelE

src/reactionsystem_conversions.jl

@@ -559,7 +559,7 @@ function nonlinear_convert_differentials_check(rs::ReactionSystem)
         # If the lhs upper level function is not a differential w.r.t. time.
         # If the contenct of the differential is not a variable (and nothing more).
         # If either of this is a case, throws the warning.
-        if Symbolics._occursin(Symbolics.is_derivative, eq.rhs) ||
+        if hasnode(Symbolics.is_derivative, eq.rhs) ||
                 !Symbolics.is_derivative(eq.lhs) ||
                 !isequal(Symbolics.operation(eq.lhs), Differential(get_iv(rs))) || 
                 (length(arguments(eq.lhs)) != 1) ||
@@ -903,4 +903,4 @@ function to_multivariate_poly(polyeqs::AbstractVector{Symbolics.BasicSymbolic{Re
     end
 
     ps
-end
+end