Handle setting TextEditingController text to null (#68638)

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

@@ -340,14 +340,14 @@ class _TextFormFieldState extends FormFieldState<String> {
     super.didChange(value);
 
     if (_effectiveController!.text != value)
-      _effectiveController!.text = value;
+      _effectiveController!.text = value ?? '';
   }
 
   @override
   void reset() {
     super.reset();
     setState(() {
-      _effectiveController!.text = widget.initialValue;
+      _effectiveController!.text = widget.initialValue ?? '';
     });
   }
 

packages/flutter/lib/src/services/text_input.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-
 import 'dart:async';
 import 'dart:io' show Platform;
 import 'dart:ui' show

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

@@ -154,7 +154,7 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
   /// [TextEditingController]; however, one should not also set [selection]
   /// in a separate statement. To change both the [text] and the [selection]
   /// change the controller's [value].
-  set text(String? newText) {
+  set text(String newText) {
     value = value.copyWith(
       text: newText,
       selection: const TextSelection.collapsed(offset: -1),

packages/flutter/test/material/text_form_field_test.dart

@@ -406,6 +406,56 @@ void main() {
     expect(find.text('initialValue'), findsOneWidget);
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/34847.
+  testWidgets('didChange resets the text field\'s value to empty when passed null', (WidgetTester tester) async {
+    await tester.pumpWidget(
+        MaterialApp(
+          home: Material(
+            child: Center(
+              child: TextFormField(
+                initialValue: null,
+              ),
+            ),
+          ),
+        )
+    );
+
+    await tester.enterText(find.byType(TextFormField), 'changedValue');
+    await tester.pump();
+    expect(find.text('changedValue'), findsOneWidget);
+
+    final FormFieldState<String> state = tester.state<FormFieldState<String>>(find.byType(TextFormField));
+    state.didChange(null);
+
+    expect(find.text('changedValue'), findsNothing);
+    expect(find.text(''), findsOneWidget);
+  });
+
+  // Regression test for https://github.com/flutter/flutter/issues/34847.
+  testWidgets('reset resets the text field\'s value to empty when intialValue is null', (WidgetTester tester) async {
+    await tester.pumpWidget(
+        MaterialApp(
+          home: Material(
+            child: Center(
+              child: TextFormField(
+                initialValue: null,
+              ),
+            ),
+          ),
+        )
+    );
+
+    await tester.enterText(find.byType(TextFormField), 'changedValue');
+    await tester.pump();
+    expect(find.text('changedValue'), findsOneWidget);
+
+    final FormFieldState<String> state = tester.state<FormFieldState<String>>(find.byType(TextFormField));
+    state.reset();
+
+    expect(find.text('changedValue'), findsNothing);
+    expect(find.text(''), findsOneWidget);
+  });
+
   // Regression test for https://github.com/flutter/flutter/issues/54472.
   testWidgets('didChange changes text fields value', (WidgetTester tester) async {
     await tester.pumpWidget(

packages/flutter/test/widgets/editable_text_test.dart

@@ -5566,6 +5566,42 @@ void main() {
     expect(focusNode.hasFocus, false);
   });
 
+  group('TextEditingController', () {
+    testWidgets('TextEditingController.text set to empty string clears field', (WidgetTester tester) async {
+      final TextEditingController controller = TextEditingController();
+      await tester.pumpWidget(
+        MaterialApp(
+          home: MediaQuery(
+            data: const MediaQueryData(devicePixelRatio: 1.0),
+            child: Directionality(
+              textDirection: TextDirection.ltr,
+              child: Center(
+                child: Material(
+                  child: EditableText(
+                    controller: controller,
+                    focusNode: focusNode,
+                    style: textStyle,
+                    cursorColor: Colors.red,
+                    backgroundCursorColor: Colors.red,
+                    keyboardType: TextInputType.multiline,
+                    onChanged: (String value) { },
+                  ),
+                ),
+              ),
+            ),
+          ),
+        ),
+      );
+
+      controller.text = '...';
+      await tester.pump();
+      expect(find.text('...'), findsOneWidget);
+
+      controller.text = '';
+      await tester.pump();
+      expect(find.text('...'), findsNothing);
+    });
+
     testWidgets('TextEditingController.clear() behavior test', (WidgetTester tester) async {
       // Regression test for https://github.com/flutter/flutter/issues/66316
       final List<MethodCall> log = <MethodCall>[];
@@ -5638,6 +5674,7 @@ void main() {
         }),
       );
     });
+  });
 
   testWidgets('autofocus:true on first frame does not throw', (WidgetTester tester) async {
     final TextEditingController controller = TextEditingController(text: testText);