wip: null safety, migration to Grid.extent

lib/models/activity.dart

@@ -85,7 +85,7 @@ class Activity {
   int? get id => _db.id;
 
   DateTime? get startTime => _db.startTime;
-  DateTime? get timeCreated => _db.timeCreated;
+  DateTime get timeCreated => _db.timeCreated!;
   DateTime? get timeStamp => _db.timeStamp;
 
   String? get event => _db.event;

lib/utils/athlete_time_series_chart.dart

@@ -61,9 +61,9 @@ class _AthleteTimeSeriesChartState extends State<AthleteTimeSeriesChart> {
 
   @override
   Widget build(BuildContext context) {
-    final List<Series<Activity, DateTime?>> data =
-        <Series<Activity, DateTime?>>[
-      Series<Activity, DateTime?>(
+    final List<Series<Activity, DateTime>> data =
+        <Series<Activity, DateTime>>[
+      Series<Activity, DateTime>(
         id: widget.activityAttr.toString(),
         colorFn: (_, __) => MaterialPalette.blue.shadeDefault,
         domainFn: (Activity activity, _) => activity.timeCreated,
@@ -71,7 +71,7 @@ class _AthleteTimeSeriesChartState extends State<AthleteTimeSeriesChart> {
             activity.getAttribute(widget.activityAttr) as num?,
         data: displayedActivities,
       ),
-      Series<Activity, DateTime?>(
+      Series<Activity, DateTime>(
         id: 'gliding_' + widget.activityAttr.toString(),
         colorFn: (_, __) => MaterialPalette.green.shadeDefault,
         domainFn: (Activity activity, _) => activity.timeCreated,
@@ -171,15 +171,12 @@ class _AthleteTimeSeriesChartState extends State<AthleteTimeSeriesChart> {
         if (selectedActivity != null)
           SizedBox(
             height: 200,
-            child: GridView.count(
+            child: GridView.extent(
               padding: const EdgeInsets.all(5),
-              crossAxisCount:
-                  MediaQuery.of(context).orientation == Orientation.landscape
-                      ? 4
-                      : 2,
-              childAspectRatio: 4,
-              crossAxisSpacing: 3,
-              mainAxisSpacing: 3,
+              maxCrossAxisExtent: 250,
+              childAspectRatio: 5,
+              crossAxisSpacing: 2,
+              mainAxisSpacing: 2,
               children: <Widget>[
                 MyButton.activity(
                   child: Text(selectedActivity!.name!),

lib/widgets/athlete_widgets/athlete_power_widget.dart

@@ -76,7 +76,7 @@ class _AthletePowerWidgetState extends State<AthletePowerWidget> {
                           sports.map<DropdownMenuItem<String>>((String? value) {
                         return DropdownMenuItem<String>(
                           value: value,
-                          child: Text(value!),
+                          child: Text(value ?? 'missing'),
                         );
                       }).toList(),
                       value: selectedSports,