fix "Null check operator used on a null value"

[rounce]

Exception

_CastError: Null check operator used on a null value
	State.setState (package:flutter/src/widgets/framework.dart:1129)
	_SnakeViewState._goRight.<fn> (package:flutter_snake_navigationbar/src/snake_view.dart:145)
	new Future.delayed.<fn> (dart:async/future.dart:423)
	_rootRun (dart:async/zone.dart:1418)
	_CustomZone.run (dart:async/zone.dart:1328)
	_CustomZone.runGuarded (dart:async/zone.dart:1236)
	_CustomZone.bindCallbackGuarded.<fn> (dart:async/zone.dart:1276)
	_rootRun (dart:async/zone.dart:1426)
	_CustomZone.run (dart:async/zone.dart:1328)
	_CustomZone.bindCallback.<fn> (dart:async/zone.dart:1260)
	Timer._createTimer.<fn> (dart:async-patch/timer_patch.dart:18)
	_Timer._runTimers (dart:isolate-patch/timer_impl.dart:398)
	_Timer._handleMessage (dart:isolate-patch/timer_impl.dart:429)
	_RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:192)

I think this issue is the same as #20

[GoltVik]

@krzeznicki @kzietek, could you help somehow?

[kzietek]

@GoltVik long time no see! Sorry it took so long. I'll merge this once I obtain access to this repo.

[vanyasem]

Hi! @kzietek have you managed to obtain the access?