add validation in faq and discount page

lib/pages/faq/faq_edit_page.dart

@@ -29,7 +29,7 @@ class _FAQEditorState extends State<FAQEditor> {
   TextEditingController _pageLabelEng = new TextEditingController();
   TextEditingController _pageLabelMm = new TextEditingController();
 
-  final _formKey = GlobalKey<FormState>();
+  final _faqFormKey = GlobalKey<FormState>();
   bool _isLoading = false;
   bool _isNew = false;
   String _pageLink = info;
@@ -57,9 +57,16 @@ class _FAQEditorState extends State<FAQEditor> {
     final snBox = InputText(
       controller: _sn,
       labelTextKey: "faq.edit.sn",
-      maxLines: 1,
       withBorder: false,
       textInputType: TextInputType.number,
+      autovalidateMode: AutovalidateMode.onUserInteraction,
+      validator: (value) {
+        if (value == null || value.isEmpty || value.length==0) {
+          return "Please insert S/N";
+        }
+
+        return null;
+      },
     );
 
     final questionEngBox = InputText(
@@ -151,7 +158,7 @@ class _FAQEditorState extends State<FAQEditor> {
               ],
             ),
             body: Form(
-              key: _formKey,
+              key: _faqFormKey,
               child: Padding(
                 padding: EdgeInsets.only(left: 24.0, right: 24.0),
                 child: ListView(
@@ -189,6 +196,10 @@ class _FAQEditorState extends State<FAQEditor> {
   }
 
   _save() async {
+    if (!_faqFormKey.currentState!.validate()) {
+      return null;
+    }
+
     setState(() {
       _isLoading = true;
     });
@@ -212,7 +223,7 @@ class _FAQEditorState extends State<FAQEditor> {
       }
       Navigator.pop(context);
     } catch (e) {
-      showMsgDialog(context, "Error", e.toString());
+      showMsgDialog(context, "Error", "Invalid S/N");
     } finally {
       setState(() {
         _isLoading = false;