Only show grade switching Warning when something is entered

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

@@ -64,6 +64,12 @@ class _GradeAverageState extends State<GradeAverage> {
             )).toList(),
 
             onSelected: (e) {
+              void switchSystem() => setState(() {
+                grades.clear();
+                gradeSystem = e;
+              });
+
+              if(grades.isNotEmpty) {
                 showDialog(context: context, builder: (context) {
                   return AlertDialog(
                     title: const Text("Notensystem wechseln"),
@@ -73,15 +79,16 @@ class _GradeAverageState extends State<GradeAverage> {
                         Navigator.of(context).pop();
                       }, child: const Text("Abbrechen")),
                       TextButton(onPressed: () {
-                      setState(() {
+                        switchSystem();
-                        grades.clear();
-                        gradeSystem = e;
-                      });
                         Navigator.of(context).pop();
                       }, child: const Text("Fortfahren")),
                     ],
                   );
                 });
+              } else {
+                switchSystem();
+              }
+
 
             },
           ),