fix nullability issues (#67443)

packages/flutter/lib/src/foundation/diagnostics.dart

@@ -1660,9 +1660,7 @@ abstract class DiagnosticsNode {
         result = toStringDeep(
             parentConfiguration: parentConfiguration, minLevel: minLevel);
       } else {
-        String? description = toDescription(parentConfiguration: parentConfiguration);
-        assert(description != null);
-        description = description!;
+        final String description = toDescription(parentConfiguration: parentConfiguration)!;
 
         if (name == null || name!.isEmpty || !showName) {
           result = description;

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

@@ -1171,7 +1171,7 @@ class FilterChip extends StatelessWidget
   @override
   final bool selected;
   @override
-  final ValueChanged<bool> onSelected;
+  final ValueChanged<bool>? onSelected;
   @override
   final double? pressElevation;
   @override

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

@@ -459,13 +459,13 @@ class _ChangeAnimation extends Animation<double> with AnimationWithParentMixin<d
 
   @override
   void removeStatusListener(AnimationStatusListener listener) {
-    if (parent != null)
+    if (controller.animation != null)
       super.removeStatusListener(listener);
   }
 
   @override
   void removeListener(VoidCallback listener) {
-    if (parent != null)
+    if (controller.animation != null)
       super.removeListener(listener);
   }
 

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

@@ -26,7 +26,7 @@ import 'theme.dart';
 export 'package:flutter/services.dart' show TextInputType, TextInputAction, TextCapitalization, SmartQuotesType, SmartDashesType;
 
 /// Signature for the [TextField.buildCounter] callback.
-typedef InputCounterWidgetBuilder = Widget Function(
+typedef InputCounterWidgetBuilder = Widget? Function(
   /// The build context for the TextField.
   BuildContext context, {
   /// The length of the string currently in the input.
@@ -880,7 +880,7 @@ class _TextFieldState extends State<TextField> with RestorationMixin implements
         && effectiveDecoration.counterText == null
         && widget.buildCounter != null) {
       final bool isFocused = _effectiveFocusNode.hasFocus;
-      final Widget builtCounter = widget.buildCounter!(
+      final Widget? builtCounter = widget.buildCounter!(
         context,
         currentLength: currentLength,
         maxLength: widget.maxLength,

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

@@ -345,7 +345,7 @@ class _TextFormFieldState extends FormFieldState<String> {
   void reset() {
     super.reset();
     setState(() {
-      _effectiveController!.text = widget.initialValue!;
+      _effectiveController!.text = widget.initialValue;
     });
   }
 

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

@@ -910,7 +910,7 @@ class _Dial extends StatefulWidget {
   final _TimePickerMode mode;
   final bool use24HourDials;
   final ValueChanged<TimeOfDay>? onChanged;
-  final VoidCallback onHourSelected;
+  final VoidCallback? onHourSelected;
 
   @override
   _DialState createState() => _DialState();
@@ -1058,7 +1058,7 @@ class _DialState extends State<_Dial> with SingleTickerProviderStateMixin {
     _animateTo(_getThetaForTime(widget.selectedTime));
     if (widget.mode == _TimePickerMode.hour) {
       if (widget.onHourSelected != null) {
-        widget.onHourSelected();
+        widget.onHourSelected!();
       }
     }
   }
@@ -1076,7 +1076,7 @@ class _DialState extends State<_Dial> with SingleTickerProviderStateMixin {
         _announceToAccessibility(context, localizations.formatDecimal(newTime.hourOfPeriod));
       }
       if (widget.onHourSelected != null) {
-        widget.onHourSelected();
+        widget.onHourSelected!();
       }
     } else {
       _announceToAccessibility(context, localizations.formatDecimal(newTime.minute));