The mh field in:

struct MethodHandleStubArguments {
  const char* adaptername;
  oopDesc* mh;
  intptr_t* saved_regs;
  intptr_t* entry_sp;
};

doesn't always point to a valid object. The oopDesc* is then implicitly converted to an oop here:

void trace_method_handle_stub_wrapper(MethodHandleStubArguments* args) {
  trace_method_handle_stub(args->adaptername,
                           args->mh,
                           args->saved_regs,
                           args->entry_sp);
}

This gets caught by my adhoc verification code that verifies oops when they are created/used.

I propose that we don't create an oop until it mh is actually used, and it has been checked that the argument should contain a valid oop.