fix: 🐛 Fixed RecalculateRootWidgetSize null check issue. (#429)

- Added `mounted` check in `addPostFrameCallback` under `recalculateRootWidgetSize` in `showcase.dart` as to ensure that the operation is only performed if the element is mounted.

Author: aditya-css

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## [2.1.1]
 - Fixed [#425](https://github.com/SimformSolutionsPvtLtd/flutter_showcaseview/issues/425) - Unhandled breaking change in v2.1.0
+- Fixed [#428](https://github.com/SimformSolutionsPvtLtd/flutter_showcaseview/issues/428) - _ShowcaseState.recalculateRootWidgetSize null value
 
 ## [2.1.0]
 - Feature [#420](https://github.com/SimformSolutionsPvtLtd/flutter_showcaseview/pull/420) - Dart SDK Upgrade

lib/src/showcase.dart

@@ -472,6 +472,7 @@ class _ShowcaseState extends State<Showcase> {
 
   void recalculateRootWidgetSize() {
     ambiguate(WidgetsBinding.instance)?.addPostFrameCallback((_) {
+      if (!mounted) return;
       final rootWidget =
           context.findRootAncestorStateOfType<State<WidgetsApp>>();
       rootRenderObject = rootWidget?.context.findRenderObject() as RenderBox?;