next_gen_ui_demo: Add dispose handler for Pictures (#1889)

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next_gen_ui_demo/pubspec.yaml

@@ -4,7 +4,7 @@ publish_to: 'none'
 version: 0.1.0
 
 environment:
-  sdk: '>=3.0.0-417.4.beta <4.0.0'
+  sdk: ^3.0.0
 
 dependencies:
   cupertino_icons: ^1.0.5