SujeetKr6

fix Widgetbinding.instance null check issue

1

This pull request will resolve the following issue - [https://github.com/victorevox/simple_tooltp/issues/41](https://github.com/victorevox/simple_tooltp/issues/41) C:/flutter/.pub-cache/hosted/pub.dartlang.org/simple_tooltip-1.2.0/lib/src/tooltip.dart:158:29: Error: Method 'addPostFrameCallback' cannot be called on 'WidgetsBinding?' because it is potentially null. - 'WidgetsBinding' is from 'package:flutter/src/widgets/binding.dart' ('/C:/flutter/packages/flutter/lib/src/widgets/binding.dart')....