这句话没错．
I don't know who he is.
这是主谓正常的．宾语从句中不缺少主语．
He is running.
I know who is running.
Mr Smith is killed in the accident.
I know who is killed in the accident.
我们可以发现,宾语从句的引导词实际上是在从句中占一个语法位置的．例句一中,who代的是人,但做的是从句宾语．
例句二和三中,who代的也是人,但它是从句实际的主语．
what在你的句中也是这样,它就是主语．
换个方式,也许好理解．
Something is happening there.
Can you see what is happening there?