fix Widgetbinding.instance null check issue

[SujeetKr6]

This pull request will resolve the following issue -
#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').
  Try calling using ?. instead.
  WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
  ^^^^^^^^^^^^^^^^^^^^

/C:/flutter/.pub-cache/hosted/pub.dartlang.org/simple_tooltip-1.2.0/lib/src/tooltip.dart:166:29: 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').
  Try calling using ?. instead.

  WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
  ^^^^^^^^^^^^^^^^^^^^
  /C:/flutter/.pub-cache/hosted/pub.dartlang.org/simple_tooltip-1.2.0/lib/src/tooltip.dart:188:29: 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').
  Try calling using ?. instead.
  WidgetsBinding.instance.addPostFrameCallback((_) {
  ^^^^^^^^^^^^^^^^^^^^

/C:/flutter/.pub-cache/hosted/pub.dartlang.org/simple_tooltip-1.2.0/lib/src/tooltip.dart:202:29: 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').
  Try calling using ?. instead.
  WidgetsBinding.instance.addPostFrameCallback((_) {

                        ^^^^^^^^^^^^^^^^^^^^
  

/C:/flutter/.pub-cache/hosted/pub.dartlang.org/simple_tooltip-1.2.0/lib/src/tooltip.dart:361:31: 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').
  Try calling using ?. instead.
  WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
  ^^^^^^^^^^^^^^^^^^^^
  /C:/flutter/.pub-cache/hosted/pub.dartlang.org/simple_tooltip-1.2.0/lib/src/ballon_transition.dart:150:29: 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').
  Try calling using ?. instead.
  WidgetsBinding.instance.addPostFrameCallback((timeStamp) {

                        ^^^^^^^^^^^^^^^^^^^^
  

/C:/flutter/.pub-cache/hosted/pub.dartlang.org/simple_tooltip-1.2.0/lib/src/balloon.dart:56:29: 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').
  Try calling using ?. instead.

  WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
  ^^^^^^^^^^^^^^^^^^^^
  /C:/flutter/.pub-cache/hosted/pub.dartlang.org/simple_tooltip-1.2.0/lib/src/balloon_positioner.dart:134:29: 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').
  Try calling using ?. instead.

  WidgetsBinding.instance.addPostFrameCallback((_) {
  ^^^^^^^^^^^^^^^^^^^^
  /C:/flutter/.pub-cache/hosted/pub.dartlang.org/simple_tooltip-1.2.0/lib/src/obfuscate_tooltip_item.dart:34:29: Error: Method 'addObserver' 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').
  Try calling using ?. instead.

  WidgetsBinding.instance.addObserver(this);
  ^^^^^^^^^^^
  /C:/flutter/.pub-cache/hosted/pub.dartlang.org/simple_tooltip-1.2.0/lib/src/obfuscate_tooltip_item.dart:43:29: 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').
  Try calling using ?. instead.
  WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
  ^^^^^^^^^^^^^^^^^^^^
  /C:/flutter/.pub-cache/hosted/pub.dartlang.org/simple_tooltip-1.2.0/lib/src/obfuscate_tooltip_item.dart:59:29: Error: Method 'removeObserver' 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').
  Try calling using ?. instead.
  WidgetsBinding.instance.removeObserver(this);
  ^^^^^^^^^^^^^^
  Restarted application in 1,111ms.

[tom-galland]

Please can we merge this in? Really struggling to build without this