Fixes for null safety

[imhafeez]

No description provided.

[prajwal27]

Will this be merged?

[EgHubs]

Alright,
I have tested this fork and it is all good with no errors,
only one thing,
when I run this simple code on Android 8.0.0

initiatedPhoneCall =
                                 FlutterPhoneState.startPhoneCall(
                                     phoneNumber);
                             print('calling $phoneNumber');
                             await initiatedPhoneCall.done;
                           print('calling ${initiatedPhoneCall.status}');

the phone app is opened and writes the number to be called but it doesn't make the call!
is it what it should do?
this is the o/p

I/flutter (13544): calling 010000000
[flutterPhoneState] Call is done: 995c8737-f6df-40fb-bc27-93315e9bb096- Removing due to PhoneCallStatus.disconnected
I/flutter (13544): calling PhoneCallStatus.disconnected
[flutterPhoneState] Finished call. Status PhoneCallStatus.disconnected
[flutterPhoneState] Received application lifecycle state change: AppLifecycleState.inactive
[flutterPhoneState] Received application lifecycle state change: AppLifecycleState.paused