Firebase优先级有时为null，即使我总是使用ServerValue.TIMESTAMP插入

Question

当我尝试调用child.getPriority()并将其转换为应该已经插入优先级为ServerValue.TIMESTAMP的子项的double时，我遇到了崩溃。事实上，我几乎可以肯定，我从不插入没有优先权的项目，但我仍然遇到这种崩溃。

java.lang.NullPointerException: Attempt to invoke virtual method 'double java.lang.Double.doubleValue()' on a null object reference
       at com.myapp.android.core.MyService$QueueListener.onDataChange(MyService.java:688)
       at com.firebase.client.core.ValueEventRegistration.fireEvent(ValueEventRegistration.java:42)
       at com.firebase.client.core.view.DataEvent.fire(DataEvent.java:45)
       at com.firebase.client.core.view.EventRaiser$1.run(EventRaiser.java:38)
       at android.os.Handler.handleCallback(Handler.java:739)
       at android.os.Handler.dispatchMessage(Handler.java:95)
       at android.os.Looper.loop(Looper.java:135)
       at android.app.ActivityThread.main(ActivityThread.java:5254)
       at java.lang.reflect.Method.invoke(Method.java)
       at java.lang.reflect.Method.invoke(Method.java:372)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)