migration to new dart version changes

lib/config/theme_data.dart

@@ -36,9 +36,9 @@ class LightThemeConfig {
 
   static const ColorScheme _colorScheme = ColorScheme(
     primary: _primary,
-    primaryVariant: _white,
+    primaryContainer: _white,
     secondary: Color(0xFFD61515),
-    secondaryVariant: _white,
+    secondaryContainer: _white,
     surface: _white,
     background: _background,
     error: Color(0xFFF00505),
@@ -116,9 +116,9 @@ class DarkThemeConfig {
 
   static const ColorScheme _colorScheme = ColorScheme(
     primary: _primary,
-    primaryVariant: _white,
+    primaryContainer: _white,
     secondary: Color(0xFFE53939),
-    secondaryVariant: _white,
+    secondaryContainer: _white,
     surface: Color(0xFF181B1F),
     background: _background,
     error: Color(0xFFF53B3B),

lib/pages/authentication/authentication_state.dart

@@ -55,5 +55,6 @@ class AuthenticationState extends CoreProvier {
       appState = AppState.failed;
       ActionSheetUtils.showAlert(AlertData(message: service.errorMessage));
     }
+    return null;
   }
 }

lib/services/network/request_helper.dart

@@ -112,5 +112,6 @@ class RequestHelper {
       }
       return result;
     }
+    return null;
   }
 }

lib/widgets/didvan/button.dart

@@ -75,5 +75,6 @@ class DidvanButton extends StatelessWidget {
         color: enabled ? color : Theme.of(context).colorScheme.disabledText,
       );
     }
+    return null;
   }
 }

lib/widgets/didvan/text_field.dart

@@ -152,6 +152,7 @@ class _DidvanTextFieldState extends State<DidvanTextField> {
     if (_error != null) {
       return Theme.of(context).colorScheme.error;
     }
+    return null;
   }
 
   Color _fillColor() {
@@ -185,6 +186,7 @@ class _DidvanTextFieldState extends State<DidvanTextField> {
         ),
       );
     }
+    return null;
   }
 
   void _onChanged(String value) {
@@ -209,6 +211,7 @@ class _DidvanTextFieldState extends State<DidvanTextField> {
         });
       }
     }
+    return null;
   }
 
   @override