Unable to add window -- token null is not valid; is your activity running?

`Process: com.example.android, PID: 10996
android.view.WindowManager$BadTokenException: Unable to add window -- token null is not valid; is your activity running?
at android.view.ViewRootImpl.setView(ViewRootImpl.java:907)
at android.view.WindowManagerGlobal.addView(WindowManagerGlobal.java:387)
at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:95)
at android.app.Dialog.show(Dialog.java:342)
at com.hbb20.CountryCodeDialog.openCountryCodeDialog(CountryCodeDialog.java:265)
at com.hbb20.CountryCodePicker.launchCountrySelectionDialog(CountryCodePicker.java:2207)
at com.hbb20.CountryCodePicker.launchCountrySelectionDialog(CountryCodePicker.java:2199)
at com.hbb20.CountryCodePicker$1.onClick(CountryCodePicker.java:147)
at android.view.View.performClick(View.java:7125)
at android.view.View.performClickInternal(View.java:7102)
at android.view.View.access$3500(View.java:801)
at android.view.View$PerformClick.run(View.java:27336)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)`

We are use ViewPager with custom view without fragment

从 stack trace 中显示的 CountryCodeDialog.openCountryCodeDialog 和 CountryCodePicker.launchCountrySelectionDialog 调用路径开始。使用在 ViewPager 内部的库和不带 fragment 的自定义 view 重现该故障，然后检查 dialog 如何获取其 activity context。在该设置下能够打开国家选择且不出现 BadTokenException，即表示完成。

由索引模型根据 Issue 内容生成。