Does not work in profile mode

**In my case it looks fine in debug mode but when I run the app as profile mode it throws an exception**

```
E/flutter ( 4423): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: Failed
assertion: boolean expression must not be null
E/flutter ( 4423): #0 _PageTurnWidgetState._captureImage (package:page_turn/src/builders/widget.dart:40)
E/flutter ( 4423): #1 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1113)
E/flutter ( 4423): #2 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1060)
E/flutter ( 4423): #3 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:968)
E/flutter ( 4423): #4 _rootRun (dart:async/zone.dart:1184)
E/flutter ( 4423): #5 _CustomZone.run (dart:async/zone.dart:1077)
E/flutter ( 4423): #6 _CustomZone.runGuarded (dart:async/zone.dart:979)
E/flutter ( 4423): #7 _invoke (dart:ui/hooks.dart:261)
E/flutter ( 4423): #8 _drawFrame (dart:ui/hooks.dart:219)
E/flutter ( 4423):
```