Migrate flutter gallery from deprecated APIs as well as remove unnecessary null-aware operator

lib/feature_discovery/feature_discovery.dart

@@ -243,7 +243,7 @@ class _FeatureDiscoveryState extends State<FeatureDiscovery>
               status = FeatureDiscoveryStatus.closed;
               openController.forward(from: 0.0);
             });
-            Overlay.of(context)?.insert(entry);
+            Overlay.of(context).insert(entry);
           });
         }
       }

lib/studies/crane/theme.dart

@@ -20,13 +20,13 @@ ThemeData _buildCraneTheme() {
     colorScheme: const ColorScheme.light().copyWith(
       primary: cranePurple800,
       secondary: craneRed700,
+      error: craneErrorOrange,
     ),
     primaryColor: cranePurple800,
     hintColor: craneWhite60,
     indicatorColor: cranePrimaryWhite,
     scaffoldBackgroundColor: cranePrimaryWhite,
     cardColor: cranePrimaryWhite,
-    errorColor: craneErrorOrange,
     highlightColor: Colors.transparent,
     textTheme: _buildCraneTextTheme(base.textTheme),
     textSelectionTheme: const TextSelectionThemeData(

lib/studies/shrine/theme.dart

@@ -26,11 +26,12 @@ ThemeData _buildShrineTheme() {
       systemOverlayStyle: SystemUiOverlayStyle.dark,
       elevation: 0,
     ),
-    colorScheme: _shrineColorScheme,
+    colorScheme: _shrineColorScheme.copyWith(
+      error: shrineErrorRed,
+    ),
     primaryColor: shrinePink100,
     scaffoldBackgroundColor: shrineBackgroundWhite,
     cardColor: shrineBackgroundWhite,
-    errorColor: shrineErrorRed,
     primaryIconTheme: _customIconTheme(base.iconTheme),
     inputDecorationTheme: const InputDecorationTheme(
       border: CutCornersBorder(

lib/themes/material_demo_theme_data.dart

@@ -7,12 +7,13 @@ import 'package:flutter/material.dart';
 
 class MaterialDemoThemeData {
   static final themeData = ThemeData(
-      colorScheme: _colorScheme,
+      colorScheme: _colorScheme.copyWith(
+        background: Colors.white,
+      ),
       canvasColor: _colorScheme.background,
       highlightColor: Colors.transparent,
       indicatorColor: _colorScheme.onPrimary,
       primaryColor: _colorScheme.primary,
-      backgroundColor: Colors.white,
       scaffoldBackgroundColor: _colorScheme.background,
       secondaryHeaderColor: _colorScheme.background,
       typography: Typography.material2018(