invalid choice dialog

lib/super.dart

@@ -51,6 +51,9 @@ class _SuperGameState extends State<SuperGame> {
     }
   }
 
+  bool _checkValidChoice(List<TTCState> game, int index) =>
+      game[index] == TTCState.empty;
+
   Widget _subGameDialog(int subGame) {
     return Dialog(
       child: Padding(
@@ -58,13 +61,33 @@ class _SuperGameState extends State<SuperGame> {
         child: Column(
           mainAxisSize: MainAxisSize.min,
           children: [
-            Text(
+            Padding(
+              padding: const EdgeInsets.only(bottom: 5),
+              child: Text(
                 "${Util.stateText(turn)} select cell",
                 style: const TextStyle(fontWeight: FontWeight.bold),
               ),
+            ),
             TTCGame(
               turn: turn,
               cellOnTapCallback: (int i) {
+                if (!_checkValidChoice(data[subGame], i)) {
+                  showDialog(
+                      context: context,
+                      builder: (context) => AlertDialog(
+                            content:
+                                Text("${data[subGame][i].name.toUpperCase()}"
+                                    " already claimed "
+                                    "[${i % 3}, ${(i / 3).floor()}]"),
+                            actions: [
+                              ElevatedButton(
+                                  onPressed: () => Navigator.pop(context),
+                                  child: const Text("Close"))
+                            ],
+                          ));
+                  return;
+                }
+
                 nextPlay = i;
                 setState(() {
                   data[subGame][i] = turn;
@@ -74,7 +97,9 @@ class _SuperGameState extends State<SuperGame> {
               },
               data: data[subGame],
             ),
-            Row(
+            Padding(
+              padding: const EdgeInsets.only(top: 5),
+              child: Row(
                 mainAxisAlignment: MainAxisAlignment.end,
                 children: [
                   ElevatedButton(
@@ -84,6 +109,7 @@ class _SuperGameState extends State<SuperGame> {
                       child: const Text("Close"))
                 ],
               ),
+            ),
           ],
         ),
       ),