wifi setup screen crashes on resume from back press in setup flow

[jiteshsingh]

reported by @muhomorr

stack trace:

E/AndroidRuntime( 1000:26775): java.lang.NullPointerException: Attempt to invoke interface method 'boolean com.android.settings.ButtonBarHandler.hasNextButton()' on a null object reference
E/AndroidRuntime( 1000:26775): 	at com.android.settings.SettingsPreferenceFragment.hasNextButton(SettingsPreferenceFragment.java:696)
E/AndroidRuntime( 1000:26775): 	at com.android.settings.network.NetworkProviderSettings.changeNextButtonState(NetworkProviderSettings.java:1204)
E/AndroidRuntime( 1000:26775): 	at com.android.settings.network.NetworkProviderSettings.onWifiEntriesChanged(NetworkProviderSettings.java:918)
E/AndroidRuntime( 1000:26775): 	at com.android.wifitrackerlib.WifiPickerTracker.lambda$notifyOnWifiEntriesChanged$43(WifiPickerTracker.java:1365)
E/AndroidRuntime( 1000:26775): 	at com.android.wifitrackerlib.WifiPickerTracker.$r8$lambda$5hTzv5FGbj2525JNVvc5MqwI0_g(WifiPickerTracker.java:0)
E/AndroidRuntime( 1000:26775): 	at com.android.wifitrackerlib.WifiPickerTracker$$ExternalSyntheticLambda44.run(R8$$SyntheticClass:0)
E/AndroidRuntime( 1000:26775): 	at android.os.Handler.handleCallback(Handler.java:959)
E/AndroidRuntime( 1000:26775): 	at android.os.Handler.dispatchMessage(Handler.java:100)
E/AndroidRuntime( 1000:26775): 	at android.os.Looper.loopOnce(Looper.java:232)
E/AndroidRuntime( 1000:26775): 	at android.os.Looper.loop(Looper.java:317)
E/AndroidRuntime( 1000:26775): 	at android.app.ActivityThread.main(ActivityThread.java:8532)
E/AndroidRuntime( 1000:26775): 	at java.lang.reflect.Method.invoke(Native Method)
E/AndroidRuntime( 1000:26775): 	at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:552)
E/AndroidRuntime( 1000:26775): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:894)

the above crash (intermittently) happens when user navigates back from date time activity in setup flow. this doesn't reproduce on emulator.

repro tips from @muhomorr for physical device;

• Press "Next" on "Connect to Wi-Fi" screen, then go back
• If it doesn't crash, do next + back again
• Note that first crash doesn't show "Settings app crashed" dialog. It'll simply go to activity before that, i.e. Welcome activity.