You are talking about something entirely different. Having weird interop is one thing. But having your internal state compromised because the language does not perform runtime checks is something entirely different.
In case of C++ this would lead to desastrous memory corruption. If all data is dynamic you can't have a safe program as data on the stack must be monomorphic or you corrupt nearby memory.
Naturally, you can defend against hostile input via excessive defensive programming (asserting against nulls, asserting against wrong types etc.). Or you simply use a strong static typed language.
