buildTextSpan method Null safety

Implementation

TextSpan buildTextSpan() {
  if (widget.obscureText) {
    String text = _value.text;
    text = widget.obscuringCharacter * text.length;
    // Reveal the latest character in an obscured field only on mobile.
    // Newer verions of iOS (iOS 15+) no longer reveal the most recently
    // entered character.
    const Set<TargetPlatform> mobilePlatforms = <TargetPlatform> {
      TargetPlatform.android, TargetPlatform.fuchsia,
    };
    final bool breiflyShowPassword = WidgetsBinding.instance.platformDispatcher.brieflyShowPassword
                                  && mobilePlatforms.contains(defaultTargetPlatform);
    if (breiflyShowPassword) {
      final int? o = _obscureShowCharTicksPending > 0 ? _obscureLatestCharIndex : null;
      if (o != null && o >= 0 && o < text.length) {
        text = text.replaceRange(o, o + 1, _value.text.substring(o, o + 1));
      }
    }
    return TextSpan(style: widget.style, text: text);
  }
  if (_placeholderLocation >= 0 && _placeholderLocation <= _value.text.length) {
    final List<_ScribblePlaceholder> placeholders = <_ScribblePlaceholder>[];
    final int placeholderLocation = _value.text.length - _placeholderLocation;
    if (_isMultiline) {
      // The zero size placeholder here allows the line to break and keep the caret on the first line.
      placeholders.add(const _ScribblePlaceholder(child: SizedBox(), size: Size.zero));
      placeholders.add(_ScribblePlaceholder(child: const SizedBox(), size: Size(renderEditable.size.width, 0.0)));
    } else {
      placeholders.add(const _ScribblePlaceholder(child: SizedBox(), size: Size(100.0, 0.0)));
    }
    return TextSpan(style: widget.style, children: <InlineSpan>[
        TextSpan(text: _value.text.substring(0, placeholderLocation)),
        ...placeholders,
        TextSpan(text: _value.text.substring(placeholderLocation)),
      ],
    );
  }

  // Read only mode should not paint text composing.
  return widget.controller.buildTextSpan(
    context: context,
    style: widget.style,
    withComposing: !widget.readOnly && _hasFocus,
  );
}