getViewTreeObserver（）.addOnGlobalLayoutListener 上的 Android 空对象引用

问：

在执行操作之前，我需要检查布局是否已完全膨胀，为此，我使用下面的代码，该代码具有一个信号量，该信号量会等待直到两个特定按钮膨胀。代码大部分时间都工作正常，但有时我收到致命异常错误：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“boolean java.util.ArrayList.add（java.lang.Object）”。有谁知道为什么以及如何解决它？下面是代码和完整日志：

    final Semaphore semaphore = new Semaphore(0);
    LayoutKBDReferencesHelper.instance().
            getLayoutKBDReferences().getBtn0().
            getViewTreeObserver().addOnGlobalLayoutListener(
                    new ViewTreeObserver.OnGlobalLayoutListener() {
                    @Override
                    public void onGlobalLayout() {
                        semaphore.release();
                    }
                });
    try {
        semaphore.acquire();
    } catch (InterruptedException e) {
        e.printStackTrace();
    } 

final Semaphore semaphore1 = new Semaphore(0);
    LayoutKBDReferencesHelper.instance().getLayoutKBDReferences().getBtn1().
            getViewTreeObserver().addOnGlobalLayoutListener(
                    new ViewTreeObserver.OnGlobalLayoutListener() {
                    @Override
                    public void onGlobalLayout() {
                        semaphore1.release();
                    }
                });
    try {
        semaphore1.acquire();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

日志：

Fatal Exception: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference
at android.view.ViewTreeObserver$CopyOnWriteArray.add(ViewTreeObserver.java:1181)
at android.view.ViewTreeObserver.addOnGlobalLayoutListener(ViewTreeObserver.java:550)
at com.example.layout.SampleLayout.showKBD(SampleLayout.java:1025)
at com.example.layout.SampleLayout.OutputCallbacks.Text(OutputCallbacks.java:108)
at com.example.layout.SampleLayout.Base.Text(SourceFile:5)
at com.example.layout.SampleLayout.LayoutTest(SourceFile)
at com.example.layout.SampleLayout.LayoutTest(SourceFile:4)
at com.example.layout.SampleLayout.LayoutTest(SourceFile:1)
at com.example.layout.SampleLayout.SampleLayout$4.run(SampleLayout.java:378)
at java.lang.Thread.run(Thread.java:764)