I'm not totally sure this fix is necessary :(

A NULL check doesn't prevent any problems with dangling pointers, since they are non-NULL.

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It might be best to just turn this into an assertion assert(tstate != NULL)

Can I solve this by

int
_PyThreadState_MustExit(PyThreadState *tstate)
{
    static inline PyThreadState *
    get_current_tstate(void)
    {
        PyThreadState *tstate = _PyThreadState_GET();
        if (tstate == NULL) {
            (void)check_interp(NULL);
            return NULL;
        }
        return check_interp(tstate->interp) ? tstate : NULL;
    }

    PyThreadState *current_tstate = get_current_tstate();
    if (current_tstate == NULL) {
        return 1;
    }

   ...
}

(code from https://github.com/python/cpython/blob/main/Python/_warnings.c#L45-L45)

No, check_interp just makes sure that it's non-NULL, not dangling. It's very difficult to safely detect if something is an invalid pointer at runtime, you should leave that to tools like valgrind. (FWIW, C doesn't support inline functions like in your snippet.)

I see. Finally, let's use assert(tstate! = NULL) Thank you for your review

FWIW, I think this should be a "skip news."
We are just enforcing a pre-condition, as @picnixz mentioned, users will not see these changes.

deleted

Yeah, this was originally conceived as a way to prevent dangling pointers, but there was some misunderstanding on the difference between a NULL and dangling pointer. It eventually just got turned into an assertion.