Fix nullability of TableRow.children (#119285)

* Fix nullablility of TableRow.children

* fix test

packages/flutter/lib/src/material/data_table.dart

@@ -967,7 +967,7 @@ class DataTable extends StatelessWidget {
     int displayColumnIndex = 0;
     if (displayCheckboxColumn) {
       tableColumns[0] = FixedColumnWidth(effectiveCheckboxHorizontalMarginStart + Checkbox.width + effectiveCheckboxHorizontalMarginEnd);
-      tableRows[0].children![0] = _buildCheckbox(
+      tableRows[0].children[0] = _buildCheckbox(
         context: context,
         checked: someChecked ? null : allChecked,
         onRowTap: null,
@@ -977,7 +977,7 @@ class DataTable extends StatelessWidget {
       );
       rowIndex = 1;
       for (final DataRow row in rows) {
-        tableRows[rowIndex].children![0] = _buildCheckbox(
+        tableRows[rowIndex].children[0] = _buildCheckbox(
           context: context,
           checked: row.selected,
           onRowTap: row.onSelectChanged == null ? null : () => row.onSelectChanged?.call(!row.selected),
@@ -1020,7 +1020,7 @@ class DataTable extends StatelessWidget {
       } else {
         tableColumns[displayColumnIndex] = const IntrinsicColumnWidth();
       }
-      tableRows[0].children![displayColumnIndex] = _buildHeadingCell(
+      tableRows[0].children[displayColumnIndex] = _buildHeadingCell(
         context: context,
         padding: padding,
         label: column.label,
@@ -1034,7 +1034,7 @@ class DataTable extends StatelessWidget {
       rowIndex = 1;
       for (final DataRow row in rows) {
         final DataCell cell = row.cells[dataColumnIndex];
-        tableRows[rowIndex].children![displayColumnIndex] = _buildDataCell(
+        tableRows[rowIndex].children[displayColumnIndex] = _buildDataCell(
           context: context,
           padding: padding,
           label: cell.child,

packages/flutter/lib/src/widgets/table.dart

@@ -32,7 +32,7 @@ export 'package:flutter/rendering.dart' show
 @immutable
 class TableRow {
   /// Creates a row in a [Table].
-  const TableRow({ this.key, this.decoration, this.children });
+  const TableRow({ this.key, this.decoration, this.children = const <Widget>[]});
 
   /// An identifier for the row.
   final LocalKey? key;
@@ -49,7 +49,7 @@ class TableRow {
   /// Children may be wrapped in [TableCell] widgets to provide per-cell
   /// configuration to the [Table], but children are not required to be wrapped
   /// in [TableCell] widgets.
-  final List<Widget>? children;
+  final List<Widget> children;
 
   @override
   String toString() {
@@ -61,9 +61,7 @@ class TableRow {
     if (decoration != null) {
       result.write('$decoration, ');
     }
-    if (children == null) {
-      result.write('child list is null');
-    } else if (children!.isEmpty) {
+    if (children.isEmpty) {
       result.write('no children');
     } else {
       result.write('$children');
@@ -127,15 +125,6 @@ class Table extends RenderObjectWidget {
     this.defaultVerticalAlignment = TableCellVerticalAlignment.top,
     this.textBaseline, // NO DEFAULT: we don't know what the text's baseline should be
   }) : assert(defaultVerticalAlignment != TableCellVerticalAlignment.baseline || textBaseline != null, 'textBaseline is required if you specify the defaultVerticalAlignment with TableCellVerticalAlignment.baseline'),
-       assert(() {
-         if (children.any((TableRow row) => row.children == null)) {
-           throw FlutterError(
-             'One of the rows of the table had null children.\n'
-             'The children property of TableRow must not be null.',
-           );
-         }
-         return true;
-       }()),
        assert(() {
          if (children.any((TableRow row1) => row1.key != null && children.any((TableRow row2) => row1 != row2 && row1.key == row2.key))) {
            throw FlutterError(
@@ -147,8 +136,8 @@ class Table extends RenderObjectWidget {
        }()),
        assert(() {
          if (children.isNotEmpty) {
-           final int cellCount = children.first.children!.length;
-           if (children.any((TableRow row) => row.children!.length != cellCount)) {
+           final int cellCount = children.first.children.length;
+           if (children.any((TableRow row) => row.children.length != cellCount)) {
              throw FlutterError(
                'Table contains irregular row lengths.\n'
                'Every TableRow in a Table must have the same number of children, so that every cell is filled. '
@@ -162,7 +151,7 @@ class Table extends RenderObjectWidget {
                               ? children.map<Decoration?>((TableRow row) => row.decoration).toList(growable: false)
                               : null {
     assert(() {
-      final List<Widget> flatChildren = children.expand<Widget>((TableRow row) => row.children!).toList(growable: false);
+      final List<Widget> flatChildren = children.expand<Widget>((TableRow row) => row.children).toList(growable: false);
       return !debugChildrenHaveDuplicateKeys(this, flatChildren, message:
         'Two or more cells in this Table contain widgets with the same key.\n'
         'Every widget child of every TableRow in a Table must have different keys. The cells of a Table are '
@@ -238,7 +227,7 @@ class Table extends RenderObjectWidget {
   RenderTable createRenderObject(BuildContext context) {
     assert(debugCheckHasDirectionality(context));
     return RenderTable(
-      columns: children.isNotEmpty ? children[0].children!.length : 0,
+      columns: children.isNotEmpty ? children[0].children.length : 0,
       rows: children.length,
       columnWidths: columnWidths,
       defaultColumnWidth: defaultColumnWidth,
@@ -254,7 +243,7 @@ class Table extends RenderObjectWidget {
   @override
   void updateRenderObject(BuildContext context, RenderTable renderObject) {
     assert(debugCheckHasDirectionality(context));
-    assert(renderObject.columns == (children.isNotEmpty ? children[0].children!.length : 0));
+    assert(renderObject.columns == (children.isNotEmpty ? children[0].children.length : 0));
     assert(renderObject.rows == children.length);
     renderObject
       ..columnWidths = columnWidths
@@ -289,7 +278,7 @@ class _TableElement extends RenderObjectElement {
       rowIndex += 1;
       return _TableElementRow(
         key: row.key,
-        children: row.children!.map<Element>((Widget child) {
+        children: row.children.map<Element>((Widget child) {
           return inflateWidget(child, _TableSlot(columnIndex++, rowIndex));
         }).toList(growable: false),
       );
@@ -347,12 +336,12 @@ class _TableElement extends RenderObjectElement {
         oldChildren = const <Element>[];
       }
       final List<_TableSlot> slots = List<_TableSlot>.generate(
-        row.children!.length,
+        row.children.length,
         (int columnIndex) => _TableSlot(columnIndex, rowIndex),
       );
       newChildren.add(_TableElementRow(
         key: row.key,
-        children: updateChildren(oldChildren, row.children!, forgottenChildren: _forgottenChildren, slots: slots),
+        children: updateChildren(oldChildren, row.children, forgottenChildren: _forgottenChildren, slots: slots),
       ));
     }
     while (oldUnkeyedRows.moveNext()) {

packages/flutter/test/widgets/table_test.dart

@@ -940,7 +940,7 @@ void main() {
   });
 
   testWidgets(
-    'Table widget requires all TableRows to have non-null children',
+    'Table widget requires all TableRows to have same number of children',
     (WidgetTester tester) async {
       FlutterError? error;
       try {
@@ -959,7 +959,7 @@ void main() {
         error = e;
       } finally {
         expect(error, isNotNull);
-        expect(error!.toStringDeep(), contains('The children property of TableRow must not be null.'));
+        expect(error!.toStringDeep(), contains('Table contains irregular row lengths.'));
       }
     },
   );