Hide 'delete all' button in AverageCalculator when no entries are set

lib/view/pages/more/gradeAverages/gradeAverage.dart

@@ -50,20 +50,23 @@ class _GradeAverageState extends State<GradeAverage> {
       appBar: AppBar(
         title: const Text("Notendurschnittsrechner"),
         actions: [
-          IconButton(onPressed: () {
-            showDialog(
-              context: context,
-              builder: (context) => ConfirmDialog(
-                title: "Zurücksetzen?",
-                content: "Alle Einträge werden entfernt.",
-                confirmButton: "Zurücksetzen",
-                onConfirm: () {
-                  grades.clear();
-                  setState(() {});
-                },
-              ),
-            );
-          }, icon: const Icon(Icons.delete_forever)),
+          Visibility(
+            visible: grades.isNotEmpty,
+            child: IconButton(onPressed: () {
+              showDialog(
+                context: context,
+                builder: (context) => ConfirmDialog(
+                  title: "Zurücksetzen?",
+                  content: "Alle Einträge werden entfernt.",
+                  confirmButton: "Zurücksetzen",
+                  onConfirm: () {
+                    grades.clear();
+                    setState(() {});
+                  },
+                ),
+              );
+            }, icon: const Icon(Icons.delete_forever)),
+          ),
           PopupMenuButton<bool>(
             enableFeedback: true,
             initialValue: gradeSystem,