fix: handle null case when uninstalling app

[krushnarout]

closes #3241

before:

ScreenRecording_10-18-2025.23-13-11_1.MP4

logs:

flutter: ----------------FIREBASE CRASHLYTICS----------------
flutter: Bad state: No element
flutter: 
#0      ListBase.firstWhere (dart:collection/list.dart:132:5)
#1      SharedPreferencesUtil.disableApp (package:omi/backend/preferences.dart:255:22)
#2      _AppDetailPageState._toggleApp (package:omi/pages/apps/app_detail/app_detail.dart:1129:13)
#3      _AppDetailPageState.build.<anonymous closure> (package:omi/pages/apps/app_detail/app_detail.dart:576:48)
#4      _AnimatedLoadingButtonState._handleOnPressed (package:omi/widgets/animated_loading_button.dart:38:27)
#5      _InkResponseState.handleTap (package:flutter/src/material/ink_well.dart:1204:21)
#6      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:345:24)
#7      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:758:11)
#8      BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:383:5)
#9      BaseTapGestureRecognizer.acceptGesture (package:flutter/src/gestures/tap.dart:353:7)
#10     GestureArenaManager.sweep (package:flutter/src/gestures/arena.dart:173:27)
#11

after:

ScreenRecording_10-18-2025.23-16-15_1.MOV