In ThrottledAsyncChecker class，it members of the completedChecks is WeakHashMap, its definition is as follows：
this.completedChecks = new WeakHashMap<>();
and one of its uses is as follows in schedule method:
after "completedChecks.containsKey(target)"， may be happen garbage collection， and result may be null.
the solution is：
this.completedChecks = new ReferenceMap(1, 1);
this.completedChecks = new HashMap<>();