getViewTreeObserver（）。Addongloballayoutlistener上的Android null对象引用

Question

我需要在进行操作之前检查布局是否已完全夸大，为此，我使用的代码具有信号量等待，直到两个细节按钮被夸大。该代码大部分时间都可以正常工作，但有时我会遇到致命异常错误：java.lang.nullpointerexception：尝试调用虚拟方法'boolean java.util.arraylist.add（java.lang.object）'null对象参考。有人知道为什么以及如何修复它吗？这是代码和完整日志：

    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)