ModalBottomSheet can be dismissed while donation is being sent

[besenkmehmet]

The user is able to dismiss the bottom sheet while sending the donation.

Debug console:

E/flutter ( 8401): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: Looking up a deactivated widget's ancestor is unsafe.
E/flutter ( 8401): At this point the state of the widget's element tree is no longer stable.
E/flutter ( 8401): To safely refer to a widget's ancestor in its dispose() method, save a reference to the ancestor by calling dependOnInheritedWidgetOfExactType() in the widget's didChangeDependencies() method.
E/flutter ( 8401): #0 Element._debugCheckStateIsActiveForAncestorLookup.
package:flutter/…/widgets/framework.dart:3906
E/flutter ( 8401): #1 Element._debugCheckStateIsActiveForAncestorLookup
package:flutter/…/widgets/framework.dart:3920
E/flutter ( 8401): #2 Element.findAncestorStateOfType
package:flutter/…/widgets/framework.dart:4039
E/flutter ( 8401): #3 Navigator.of
package:flutter/…/widgets/navigator.dart:2179
E/flutter ( 8401): #4 Navigator.pop
package:flutter/…/widgets/navigator.dart:2068
E/flutter ( 8401): #5 CustomModalSheet._donate
package:flutter_hackathon_bread_donate/…/home/custom_modal_sheet.dart:32
E/flutter ( 8401):
E/flutter ( 8401): #6 CustomModalSheet.build..
package:flutter_hackathon_bread_donate/…/home/custom_modal_sheet.dart:67
E/flutter ( 8401): #7 _InkResponseState._handleTap
package:flutter/…/material/ink_well.dart:993
E/flutter ( 8401): #8 _InkResponseState.build.
package:flutter/…/material/ink_well.dart:1111
E/flutter ( 8401): #9 GestureRecognizer.invokeCallback
package:flutter/…/gestures/recognizer.dart:183
E/flutter ( 8401): #10 TapGestureRecognizer.handleTapUp
package:flutter/…/gestures/tap.dart:598
E/flutter ( 8401): #11 BaseTapGestureRecognizer._checkUp
package:flutter/…/gestures/tap.dart:287
E/flutter ( 8401): #12 BaseTapGestureRecognizer.acceptGesture
package:flutter/…/gestures/tap.dart:259
E/flutter ( 8401): #13 GestureArenaManager.sweep
package:flutter/…/gestures/arena.dart:157
E/flutter ( 8401): #14 GestureBinding.handleEvent
package:flutter/…/gestures/binding.dart:362
E/flutter ( 8401): #15 GestureBinding.dispatchEvent
package:flutter/…/gestures/binding.dart:338
E/flutter ( 8401): #16 RendererBinding.dispatchEvent
package:flutter/…/rendering/binding.dart:267
E/flutter ( 8401): #17 GestureBinding._handlePointerEvent
package:flutter/…/gestures/binding.dart:295
E/flutter ( 8401): #18 GestureBinding._flushPointerEventQueue
package:flutter/…/gestures/binding.dart:240
E/flutter ( 8401): #19 GestureBinding._handlePointerDataPacket
package:flutter/…/gestures/binding.dart:213
E/flutter ( 8401): #20 _rootRunUnary (dart:async/zone.dart:1206:13)
E/flutter ( 8401): #21 _CustomZone.runUnary (dart:async/zone.dart:1100:19)
E/flutter ( 8401): #22 _CustomZone.runUnaryGuarded (dart:async/zone.dart:1005:7)
E/flutter ( 8401): #23 _invoke1 (dart:ui/hooks.dart:265:10)
E/flutter ( 8401): #24 _dispatchPointerDataPacket (dart:ui/hooks.dart:174:5)