Fix #49: allow SmartSelect.single without an initial value

example/lib/features_brightness.dart

@@ -23,7 +23,7 @@ class _FeaturesBrightnessState extends State<FeaturesBrightness> {
       selectedValue:
           ThemePatrol.of(context)?.themeMode?.index ?? ThemeMode.system.index,
       onChange: (selected) {
-        ThemePatrol.of(context)?.setMode(ThemeMode.values[selected.value]);
+        ThemePatrol.of(context)?.setMode(ThemeMode.values[selected.value!]);
       },
       modalType: S2ModalType.bottomSheet,
       modalHeader: false,
@@ -47,7 +47,7 @@ class _FeaturesBrightnessState extends State<FeaturesBrightness> {
       },
       tileBuilder: (context, state) {
         return IconButton(
-          icon: Icon(modes[state.selected.value].meta),
+          icon: Icon(modes[state.selected.value!].meta),
           onPressed: state.showModal,
         );
       },

example/lib/features_color.dart

@@ -24,7 +24,7 @@ class _FeaturesColorState extends State<FeaturesColor> {
       title: 'Color',
       selectedValue: _themeColor,
       onChange: (selected) {
-        setState(() => _themeColor = selected.value);
+        setState(() => _themeColor = selected.value!);
         ThemePatrol.of(context)?.setColor(_themeColor);
       },
       choiceItems: colors,

example/lib/features_theme.dart

@@ -92,7 +92,7 @@ class _FeaturesThemeState extends State<FeaturesTheme> {
             title: 'Color',
             selectedValue: _themeColor,
             onChange: (selected) {
-              setState(() => _themeColor = selected.value);
+              setState(() => _themeColor = selected.value!);
               ThemePatrol.of(context)?.setColor(_themeColor);
             },
             choiceItems: S2Choice.listFrom<Color, Color>(

lib/src/widget/s2_single_state.dart

@@ -12,7 +12,7 @@ class S2SingleState<T> extends S2State<T> {
 
   /// State of choice(s) selection in the modal
   @override
-  S2SingleSelection<T>? selection;
+  S2SingleSelection<T?>? selection;
 
   @override
   void onChange() {
@@ -54,12 +54,12 @@ class S2SingleState<T> extends S2State<T> {
   }
 
   @override
-  S2Validation<S2SingleChosen<T>>? get validation {
+  S2Validation<S2SingleChosen<T?>>? get validation {
     return widget.singleValidation;
   }
 
   @override
-  S2Validation<S2SingleChosen<T>>? get modalValidation {
+  S2Validation<S2SingleChosen<T?>>? get modalValidation {
     return widget.singleModalValidation ?? validation;
   }
 
@@ -77,7 +77,7 @@ class S2SingleState<T> extends S2State<T> {
   @override
   void resolveSelection() async {
     // set the initial selection
-    selection = S2SingleSelection<T>(
+    selection = S2SingleSelection<T?>(
       initial: selected.choice,
       validation: modalValidation,
     )
@@ -206,5 +206,5 @@ class S2SingleState<T> extends S2State<T> {
   Widget? choiceSelector(List<S2Choice<T>>? values) => null;
 
   @override
-  S2SingleSelected<T> get selected => widget.singleSelected!;
+  S2SingleSelected<T?> get selected => widget.singleSelected!;
 }

lib/src/widget/s2_state.dart

@@ -14,10 +14,10 @@ import '../text_error.dart';
 /// Smart Select State
 abstract class S2State<T> extends State<SmartSelect<T>> {
   /// State of the selected choice(s)
-  S2Selected<T> get selected;
+  S2Selected<T?> get selected;
 
   /// State of choice(s) selection in the modal
-  covariant S2Selection<T>? selection;
+  covariant S2Selection<T?>? selection;
 
   /// State of the choice items
   S2Choices<T>? choices;

lib/src/widget/smart_select.dart

@@ -34,16 +34,16 @@ class SmartSelect<T> extends StatefulWidget {
   final bool isMultiChoice;
 
   /// Initial selected choice
-  final S2SingleSelected<T>? singleSelected;
+  final S2SingleSelected<T?>? singleSelected;
 
   /// A function used to validate the selected choice
-  final S2Validation<S2SingleChosen<T>>? singleValidation;
+  final S2Validation<S2SingleChosen<T?>>? singleValidation;
 
   /// Modal validation of single choice widget
-  final S2Validation<S2SingleChosen<T>>? singleModalValidation;
+  final S2Validation<S2SingleChosen<T?>>? singleModalValidation;
 
   /// Called when value changed in single choice widget
-  final ValueChanged<S2SingleSelected<T>>? singleOnChange;
+  final ValueChanged<S2SingleSelected<T?>>? singleOnChange;
 
   /// Called when selection has been made in single choice widget
   final S2ChoiceSelect<S2SingleState<T>, S2Choice<T>>? singleOnSelect;
@@ -331,17 +331,17 @@ class SmartSelect<T> extends StatefulWidget {
     Key? key,
     String? title,
     String placeholder = 'Select one',
-    required T selectedValue,
+    T? selectedValue,
     S2Choice<T>? selectedChoice,
-    S2SingleSelectedResolver<T>? selectedResolver,
-    ValueChanged<S2SingleSelected<T>>? onChange,
+    S2SingleSelectedResolver<T?>? selectedResolver,
+    ValueChanged<S2SingleSelected<T?>>? onChange,
     S2ChoiceSelect<S2SingleState<T>, S2Choice<T>>? onSelect,
     S2ModalOpen<S2SingleState<T>>? onModalOpen,
     S2ModalClose<S2SingleState<T>>? onModalClose,
     S2ModalWillOpen<S2SingleState<T>>? onModalWillOpen,
     S2ModalWillClose<S2SingleState<T>>? onModalWillClose,
-    S2Validation<S2SingleChosen<T>>? validation,
-    S2Validation<S2SingleChosen<T>>? modalValidation,
+    S2Validation<S2SingleChosen<T?>>? validation,
+    S2Validation<S2SingleChosen<T?>>? modalValidation,
     List<S2Choice<T>>? choiceItems,
     S2ChoiceLoader<T>? choiceLoader,
     S2SingleBuilder<T>? builder,
@@ -406,7 +406,7 @@ class SmartSelect<T> extends StatefulWidget {
       choiceItems: choiceItems,
       choiceLoader: choiceLoader,
       isMultiChoice: false,
-      singleSelected: S2SingleSelected<T>(
+      singleSelected: S2SingleSelected<T?>(
         value: selectedValue,
         choice: selectedChoice,
         resolver: selectedResolver,