PictureInPicture - Java - Sample Application Crashes

The sample application crashes when checkout the repository and run the project PictureInPicture in emulator with following crash log:

```
2021-06-07 16:44:27.046 9271-9271/com.example.android.pictureinpicture E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.pictureinpicture, PID: 9271
java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.core.view.WindowInsetsControllerCompat.show(int)' on a null object reference
at com.example.android.pictureinpicture.MainActivity.adjustFullScreen(MainActivity.java:313)
at com.example.android.pictureinpicture.MainActivity.onWindowFocusChanged(MainActivity.java:241)
at androidx.appcompat.view.WindowCallbackWrapper.onWindowFocusChanged(WindowCallbackWrapper.java:124)
at com.android.internal.policy.DecorView.onWindowFocusChanged(DecorView.java:1701)
at android.view.View.dispatchWindowFocusChanged(View.java:14448)
at android.view.ViewGroup.dispatchWindowFocusChanged(ViewGroup.java:1491)
at android.view.ViewRootImpl.handleWindowFocusChanged(ViewRootImpl.java:3216)
at android.view.ViewRootImpl.access$1100(ViewRootImpl.java:188)
at android.view.ViewRootImpl$ViewRootHandler.handleMessage(ViewRootImpl.java:4999)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:216)
at android.app.ActivityThread.main(ActivityThread.java:7464)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:549)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:955)
2021-06-07 16:44:27.233 9271-9291/com.example.android.pictureinpicture W/MediaPlayerNative: info/warning (3, 0)
```

Tried on Emulator with `Android 11.0` and `Android 9.0`
Android Studio 4.1.3
Build #AI-201.8743.12.41.7199119, built on March 11, 2021
Runtime version: 1.8.0_242-release-1644-b3-6915495 x86_64
VM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
macOS 10.16
GC: ParNew, ConcurrentMarkSweep
Memory: 4012M
Cores: 12
Registry: ide.new.welcome.screen.force=true, external.system.auto.import.disabled=true
Non-Bundled Plugins: com.intellij.marketplace, org.jetbrains.kotlin

Kindly ignore if it's a mistake or duplicate issue!
Let me know if you need any more information.

Thanks.

Start in MainActivity.java at adjustFullScreen on line 313, reached from onWindowFocusChanged on line 241. Reproduce the crash by checking out the PictureInPicture sample and running it in an Android 11 or Android 9 emulator. Done means the sample starts and changes window focus without the reported NullPointerException.

Written by the indexing model from the issue text.