better error handling when trying to pop the only page

[slovnicki]

Instead of

RangeError (index): Invalid value: Valid value range is empty: -1

When the exception was thrown, this was the stack: 
#0      List.[] (dart:core-patch/growable_array.dart:254:60)
#1      List.removeLast (dart:core-patch/growable_array.dart:325:20)
#2      _defaultOnPopPage (package:beamer/src/beam_page.dart:33:45)
#3      BeamerDelegate.build.<anonymous closure>.<anonymous closure> (package:beamer/src/beamer_delegate.dart:582:53)
#4      NavigatorState.pop (package:flutter/src/widgets/navigator.dart:5020:28)
...
Handler: "onTap"
Recognizer: TapGestureRecognizer#3330a
  debugOwner: GestureDetector
  state: ready
  won arena
  finalPosition: Offset(121.1, 324.0)
  finalLocalPosition: Offset(121.1, 32.0)
  button: 1
  sent tap down
==============

https://github.com/slovnicki/beamer/issues/272

[rayliverified]

Just ran into this error as well. In my case, it happened because of Navigator.maybePop()

I/flutter (26681): #0      List.[] (dart:core-patch/growable_array.dart:254:60)
I/flutter (26681): #1      List.removeLast (dart:core-patch/growable_array.dart:325:20)
I/flutter (26681): #2      BeamPage.defaultOnPopPage (package:beamer/src/beam_page.dart:47:47)
I/flutter (26681): #3      BeamerDelegate.build.<anonymous closure>.<anonymous closure> (package:beamer/src/beamer_delegate.dart:620:55)
I/flutter (26681): #4      NavigatorState.pop (package:flutter/src/widgets/navigator.dart:5074:28)
I/flutter (26681): #5      Navigator.pop (package:flutter/src/widgets/navigator.dart:2613:27)
I/flutter (26681): #6      BackHelper.build.<anonymous closure> (package:nocd/utils/back_helper.dart:39:21)
I/flutter (26681): #7      ModalRoute.willPop (package:flutter/src/widgets/routes.dart:1360:25)
I/flutter (26681): #8      NavigatorState.maybePop (package:flutter/src/widgets/navigator.dart:5020:67)
I/flutter (26681): #9      Navigator.maybePop (package:flutter/src/widgets/navigator.dart:2563:34)

This also raises the question of whether the correct response is returned to maybePop?