now if user change end time into value before start time it prevent it with dialog information showing the error and if the init start date is null fill it with the needed value

lib/pages/visitor_password/bloc/visitor_password_bloc.dart

@@ -21,7 +21,6 @@ import 'package:syncrow_web/utils/snack_bar.dart';
 
 class VisitorPasswordBloc
     extends Bloc<VisitorPasswordEvent, VisitorPasswordState> {
-
   VisitorPasswordBloc() : super(VisitorPasswordInitial()) {
     on<SelectUsageFrequency>(selectUsageFrequency);
     on<FetchDevice>(_onFetchDevice);
@@ -87,6 +86,9 @@ class VisitorPasswordBloc
     SelectTimeVisitorPassword event,
     Emitter<VisitorPasswordState> emit,
   ) async {
+    // Ensure expirationTimeTimeStamp has a value
+    effectiveTimeTimeStamp ??= DateTime.now().millisecondsSinceEpoch ~/ 1000;
+
     final DateTime? picked = await showDatePicker(
       context: event.context,
       initialDate: DateTime.now(),
@@ -94,7 +96,8 @@ class VisitorPasswordBloc
       lastDate: DateTime.now().add(const Duration(days: 5095)),
     );
 
-    if (picked != null) {
+    if (picked == null) return;
+
     final TimeOfDay? timePicked = await showTimePicker(
       context: event.context,
       initialTime: TimeOfDay.now(),
@@ -105,18 +108,14 @@ class VisitorPasswordBloc
               primary: ColorsManager.primaryColor,
               onSurface: Colors.black,
             ),
-              buttonTheme: const ButtonThemeData(
-                colorScheme: ColorScheme.light(
-                  primary: Colors.green,
-                ),
-              ),
           ),
           child: child!,
         );
       },
     );
 
-      if (timePicked != null) {
+    if (timePicked == null) return;
+
     final selectedDateTime = DateTime(
       picked.year,
       picked.month,
@@ -124,57 +123,98 @@ class VisitorPasswordBloc
       timePicked.hour,
       timePicked.minute,
     );
-
+    final selectedTimestamp = selectedDateTime.millisecondsSinceEpoch ~/ 1000;
-        final selectedTimestamp =
+    final currentTimestamp = DateTime.now().millisecondsSinceEpoch ~/ 1000;
-            selectedDateTime.millisecondsSinceEpoch ~/ 1000;
 
     if (event.isStart) {
+      // START TIME VALIDATION
       if (expirationTimeTimeStamp != null &&
           selectedTimestamp > expirationTimeTimeStamp!) {
-            CustomSnackBar.displaySnackBar(
-              'Effective Time cannot be later than Expiration Time.',
-            );
-            return;
-          }
-          if(selectedTimestamp < DateTime.now().millisecondsSinceEpoch ~/ 1000) {
-            if(selectedTimestamp < DateTime.now().millisecondsSinceEpoch ~/ 1000) {
         await showDialog<void>(
           context: event.context,
           builder: (context) => AlertDialog(
-                  title: const Text('Effective Time cannot be earlier than current time.'),
+            title: const Text(
+              'Effective Time cannot be later than Expiration Time.',
+            ),
             actionsAlignment: MainAxisAlignment.center,
-                  content: 
+            content: FilledButton(
-                    FilledButton(
               onPressed: () {
                 Navigator.of(event.context).pop();
-                        add(SelectTimeVisitorPassword(context: event.context, isStart: true, isRepeat: false));
+                add(SelectTimeVisitorPassword(
+                  context: event.context,
+                  isStart: true,
+                  isRepeat: false,
+                ));
               },
               child: const Text('OK'),
             ),
-                  
           ),
         );
-            }
         return;
       }
+
+      if (selectedTimestamp < currentTimestamp) {
+        await showDialog<void>(
+          context: event.context,
+          builder: (context) => AlertDialog(
+            title: const Text(
+              'Effective Time cannot be earlier than current time.',
+            ),
+            actionsAlignment: MainAxisAlignment.center,
+            content: FilledButton(
+              onPressed: () {
+                Navigator.of(event.context).pop();
+                add(SelectTimeVisitorPassword(
+                  context: event.context,
+                  isStart: true,
+                  isRepeat: false,
+                ));
+              },
+              child: const Text('OK'),
+            ),
+          ),
+        );
+        return;
+      }
+
+      // Save effective time
       effectiveTimeTimeStamp = selectedTimestamp;
       startTimeAccess = selectedDateTime.toString().split('.').first;
     } else {
+      // END TIME VALIDATION
       if (effectiveTimeTimeStamp != null &&
           selectedTimestamp < effectiveTimeTimeStamp!) {
-            CustomSnackBar.displaySnackBar(
+        await showDialog<void>(
+          context: event.context,
+          builder: (context) => AlertDialog(
+            title: const Text(
               'Expiration Time cannot be earlier than Effective Time.',
+            ),
+            actionsAlignment: MainAxisAlignment.center,
+            content: FilledButton(
+              onPressed: () {
+                Navigator.of(event.context).pop();
+                add(SelectTimeVisitorPassword(
+                  context: event.context,
+                  isStart: false,
+                  isRepeat: false,
+                ));
+              },
+              child: const Text('OK'),
+            ),
+          ),
         );
         return;
       }
+
+      // Save expiration time
       expirationTimeTimeStamp = selectedTimestamp;
       endTimeAccess = selectedDateTime.toString().split('.').first;
     }
+
     emit(ChangeTimeState());
     emit(VisitorPasswordInitial());
   }
-    }
-  }
 
   bool toggleRepeat(
       ToggleRepeatEvent event, Emitter<VisitorPasswordState> emit) {