check null safety for my data table

lib/pages/widgets/my_data_table.dart

@@ -75,7 +75,7 @@ class MyDataRow {
     this.key,
     this.selected = false,
     this.onSelectChanged,
-    @required this.cells,
+    required this.cells,
   }) : assert(cells != null);
 
   /// Creates the configuration for a row of a [MyDataTable], deriving
@@ -83,10 +83,10 @@ class MyDataRow {
   ///
   /// The [cells] argument must not be null.
   MyDataRow.byIndex({
-    int index,
+    int index = 0,
     this.selected = false,
     this.onSelectChanged,
-    @required this.cells,
+    required this.cells,
   })  : assert(cells != null),
         key = ValueKey<int>(index);
 
@@ -96,7 +96,7 @@ class MyDataRow {
   /// remain on the right row visually.
   ///
   /// If the table never changes once created, no key is necessary.
-  final LocalKey key;
+  final LocalKey? key;
 
   /// Called when the user selects or unselects a selectable row.
   ///
@@ -111,7 +111,7 @@ class MyDataRow {
   /// A row whose [onSelectChanged] callback is null is ignored for
   /// the purposes of determining the state of the "all" checkbox,
   /// and its checkbox is disabled.
-  final ValueChanged<bool> onSelectChanged;
+  final ValueChanged<bool>? onSelectChanged;
 
   /// Whether the row is selected.
   ///
@@ -193,7 +193,7 @@ class MyDataCell {
   /// If non-null, tapping the cell will call this callback. If
   /// null, tapping the cell will attempt to select the row (if
   /// [MyDataRow.onSelectChanged] is provided).
-  final VoidCallback onTap;
+  final VoidCallback? onTap;
 
   bool get _debugInteractive => onTap != null;
 }
@@ -309,8 +309,8 @@ class MyDataTable extends StatelessWidget {
   /// the sort order is ascending, this should be true (the default),
   /// otherwise it should be false.
   MyDataTable({
-    Key key,
-    @required this.columns,
+    Key? key,
+    required this.columns,
     this.sortColumnIndex,
     this.sortAscending = true,
     this.onSelectAll,
@@ -339,8 +339,8 @@ class MyDataTable extends StatelessWidget {
   /// The configuration and labels for the columns in the table.
   final List<MyDataColumn> columns;
 
-  final Decoration oddLine;
-  final Decoration evenLine;
+  final Decoration? oddLine;
+  final Decoration? evenLine;
 
   /// The current primary sort key's column.
   ///
@@ -353,7 +353,7 @@ class MyDataTable extends StatelessWidget {
   ///
   /// When this is null, it implies that the table's sort order does
   /// not correspond to any of the columns.
-  final int sortColumnIndex;
+  final int? sortColumnIndex;
 
   /// Whether the column mentioned in [sortColumnIndex], if any, is sorted
   /// in ascending order.
@@ -376,7 +376,7 @@ class MyDataTable extends StatelessWidget {
   /// To control whether a particular row is selectable or not, see
   /// [MyDataRow.onSelectChanged]. This callback is only relevant if any
   /// row is selectable.
-  final ValueSetter<bool> onSelectAll;
+  final ValueSetter<bool>? onSelectAll;
 
   /// The height of each row (excluding the row that contains column headings).
   ///
@@ -413,11 +413,10 @@ class MyDataTable extends StatelessWidget {
   // non-numeric, if there is exactly one, otherwise null.
   final int _onlyTextColumn;
   static int _initOnlyTextColumn(List<MyDataColumn> columns) {
-    int result;
+    int result = 0;
     for (int index = 0; index < columns.length; index += 1) {
       final MyDataColumn column = columns[index];
       if (!column.numeric) {
-        if (result != null) return null;
         result = index;
       }
     }
@@ -433,11 +432,11 @@ class MyDataTable extends StatelessWidget {
 
   void _handleSelectAll(bool checked) {
     if (onSelectAll != null) {
-      onSelectAll(checked);
+      onSelectAll!(checked);
     } else {
       for (MyDataRow row in rows) {
         if ((row.onSelectChanged != null) && (row.selected != checked))
-          row.onSelectChanged(checked);
+          row.onSelectChanged!(checked);
       }
     }
   }
@@ -613,8 +612,8 @@ class MyDataTable extends StatelessWidget {
     final bool showCheckboxColumn = false;
     final bool allChecked = false;
 
-    final List<TableColumnWidth> tableColumns =
-        List<TableColumnWidth>(columns.length + (showCheckboxColumn ? 1 : 0));
+    final List<TableColumnWidth> tableColumns = (columns.length +
+        (showCheckboxColumn ? 1 : 0)) as List<TableColumnWidth>;
     final List<TableRow> tableRows = List<TableRow>.generate(
       rows.length + 1, // the +1 is for the header row
       (int index) {
@@ -745,7 +744,7 @@ class MyDataTable extends StatelessWidget {
 class TableRowInkWell extends InkResponse {
   /// Creates an ink well for a table row.
   const TableRowInkWell({
-    Key key,
+    Key? key,
     Widget child,
     GestureTapCallback onTap,
     GestureTapCallback onDoubleTap,
@@ -799,7 +798,7 @@ class TableRowInkWell extends InkResponse {
 
 class _SortArrow extends StatefulWidget {
   const _SortArrow({
-    Key key,
+    Key? key,
     this.visible,
     this.down,
     this.duration,
@@ -816,14 +815,14 @@ class _SortArrow extends StatefulWidget {
 }
 
 class _SortArrowState extends State<_SortArrow> with TickerProviderStateMixin {
-  AnimationController _opacityController;
-  Animation<double> _opacityAnimation;
+  late AnimationController _opacityController;
+  late Animation<double> _opacityAnimation;
 
-  AnimationController _orientationController;
-  Animation<double> _orientationAnimation;
+  late AnimationController _orientationController;
+  late Animation<double> _orientationAnimation;
   double _orientationOffset = 0.0;
 
-  bool _down;
+  late bool _down;
 
   static final Animatable<double> _turnTween =
       Tween<double>(begin: 0.0, end: math.pi)
@@ -869,7 +868,7 @@ class _SortArrowState extends State<_SortArrow> with TickerProviderStateMixin {
   void didUpdateWidget(_SortArrow oldWidget) {
     super.didUpdateWidget(oldWidget);
     bool skipArrow = false;
-    final bool newDown = widget.down ?? _down;
+    final bool newDown = widget.down;
     if (oldWidget.visible != widget.visible) {
       if (widget.visible &&
           (_opacityController.status == AnimationStatus.dismissed)) {