我无法解决 null 检查运算符用于 null 值错误

Question

Uint8List? _file;

SizedBox(
                  height: 45,
                  width: 45,
                  child: AspectRatio(
                    aspectRatio: 487 / 451,
                    child: Container(
                      decoration: BoxDecoration(
                          image: DecorationImage(
                              image: MemoryImage(_file!),
                              fit: BoxFit.fill,
                              alignment: FractionalOffset.topCenter)),
                    ),
                  ),
                ),

Null check operator used on a null value. I know the reason for this error but I couldn't figure out how to solve it.

With Uint8List? the value I defined as 'This value may be null', with _file. I'm trying to call it with 'This value is not null'. This is the cause of the error but I couldn't figure out how to fix it.

Answer 1

It would be better to do a null check 1st.

if (_file != null)
  SizedBox(
    height: 45,
    width: 45,
    child: AspectRatio(
      aspectRatio: 487 / 451,
      child: Container(
        decoration: BoxDecoration(
            image: DecorationImage(
                image: MemoryImage(_file!),
                fit: BoxFit.fill,
                alignment: FractionalOffset.topCenter)),
      ),
    ),
  ),

Or

image: _file != null
    ? DecorationImage(
        image: MemoryImage(_file),
        fit: BoxFit.fill,
        alignment: FractionalOffset.topCenter,
      )
    : null,