我正在使用flutter创建一个登录页面，如果没有验证器错误，我的输入字段如下所示：InputImage
但是如果验证器返回一条消息，该消息将像这样移动我的输入字段的图标和占位符：
InputImage2
我该怎么修呢？
输入小部件的代码如下所示：

class InputFieldWidget extends StatelessWidget {
  final TextEditingController cnt;
  final String placeholder;
  final Widget? icon;
  final GlobalKey formKey;

  const InputFieldWidget(
      {required this.cnt, required this.placeholder, required this.icon, required this.formKey} );

  @override
  Widget build(BuildContext context) {
    return Container(
      decoration: BoxDecoration(
        borderRadius: BorderRadius.circular(10),
        color: Palette.lightGrey,
      ),
      height: 50,
      margin: EdgeInsets.all(10),
      child: Form(
        key: formKey,
        child: TextFormField(
          validator: (value){
            if(value == null || value.isEmpty) {
              return "      $placeholder is not valid!";
            }
          },
          cursorColor: Palette.lightPurple,
          controller: cnt,
          decoration: InputDecoration(
              border: InputBorder.none,
              contentPadding: const EdgeInsets.symmetric(vertical: 15),
              prefixIcon: icon,
              hintText: placeholder,
              focusColor: Palette.lightPurple),
        ),
      ),
    );
  }
}