Fix buildCounter returns a widget when set to return null. (#45749)

When buildCounter returns null it no longer produces a widget that takes up space.

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

@@ -671,6 +671,9 @@ class TextField extends StatefulWidget {
   /// }
   /// ```
   /// {@end-tool}
+  ///
+  /// If buildCounter returns null, then no counter and no Semantics widget will
+  /// be created at all.
   final InputCounterWidgetBuilder buildCounter;
 
   /// {@macro flutter.widgets.editableText.scrollPhysics}
@@ -771,16 +774,20 @@ class _TextFieldState extends State<TextField> implements TextSelectionGestureDe
         && effectiveDecoration.counterText == null
         && widget.buildCounter != null) {
       final bool isFocused = _effectiveFocusNode.hasFocus;
-      counter = Semantics(
-        container: true,
-        liveRegion: isFocused,
-        child: widget.buildCounter(
-          context,
-          currentLength: currentLength,
-          maxLength: widget.maxLength,
-          isFocused: isFocused,
-        ),
+      final Widget builtCounter = widget.buildCounter(
+        context,
+        currentLength: currentLength,
+        maxLength: widget.maxLength,
+        isFocused: isFocused,
       );
+      // If buildCounter returns null, don't add a counter widget to the field.
+      if (builtCounter != null) {
+        counter = Semantics(
+          container: true,
+          liveRegion: isFocused,
+          child: builtCounter,
+        );
+      }
       return effectiveDecoration.copyWith(counter: counter);
     }
 

packages/flutter/test/material/text_field_test.dart

@@ -7402,4 +7402,34 @@ void main() {
     await tester.pump();
     expect(focusNode3.hasPrimaryFocus, isTrue);
   });
+
+  testWidgets("A buildCounter that returns null doesn't affect the size of the TextField", (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/44909
+
+    final GlobalKey textField1Key = GlobalKey();
+    final GlobalKey textField2Key = GlobalKey();
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: Column(
+            children: <Widget>[
+              TextField(key: textField1Key),
+              TextField(
+                key: textField2Key,
+                maxLength: 1,
+                buildCounter: (BuildContext context, {int currentLength, bool isFocused, int maxLength}) => null,
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpAndSettle();
+    final Size textFieldSize1 = tester.getSize(find.byKey(textField1Key));
+    final Size textFieldSize2 = tester.getSize(find.byKey(textField2Key));
+
+    expect(textFieldSize1, equals(textFieldSize2));
+  });
 }