As the comment above cpu_switch_context_exit notes:
sched_active_thread is not valid when cpu_switch_context_exit() is called.
Unfortunately, thread_yield(), which is called directly by
cpu_switch_context_exit(), uses sched_active_thread possibly resulting
in a null pointer dereference.
Solution: Trigger a software interrupt to perform a context switch and
let sched_run() determine the next valid thread from there.
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