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

Start with CountryCodeDialog.openCountryCodeDialog and the CountryCodePicker.launchCountrySelectionDialog call path shown in the stack trace. Reproduce the failure with the library used inside a ViewPager and a custom view without a fragment, then inspect how the dialog obtains its activity context. Done means country selection opens without the BadTokenException in that setup.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.