What can I do about this PickedFile error?

It is because you are assigning a nullable type (PickedFile?) to a non-nullable type (PickedFile).

What you can do:

Make it nullable

If it is fine to use a nullable type, just make picture nullable:

void _openCamera() async {
  PickedFile? picture = await ImagePicker.getImage(source: ImageSource.camera);
  images.add(picture);
  Navigator.pop(context);
}

Use the null-check operator

If you are sure that ImagePicker.getImage() is not going to return null, you can use the null-check operator (!).

void _openCamera() async {
  PickedFile picture = await ImagePicker.getImage(source: ImageSource.camera)!; // <- Notice the `!`
  images.add(picture);
  Navigator.pop(context);
}

Make it nullable and filter it if it is null

If ImagePicker.getImage() can return null and you don't want your images list to contain null values. You can make it nullable and filter it if it is null:

void _openCamera() async {
  PickedFile? picture = await ImagePicker.getImage(source: ImageSource.camera);
  if (picture != null) {
    images.add(picture);
  }
  Navigator.pop(context);
}

/// You can use this code hope this will work for you, Thanks

    final picker = ImagePicker();
    ///pop-up to choose camera gallery while selecting image
      Future<void> _showChoiceDialog(BuildContext context) {
        return showDialog(
            context: context,
            builder: (BuildContext context) {
              return AlertDialog(
                title: Text(
                  "Choose option",
                  style: TextStyle(color: AppColors.hotelListDarkBlue),
                ),
                content: SingleChildScrollView(
                  child: ListBody(
                    children: [
                      Divider(
                        height: 1,
                        color: AppColors.baseLightBlueColor,
                      ),
                      ListTile(
                        onTap: () {
                          Navigator.pop(context, _pickImage(ImageSource.gallery,context));
                        },
                        title: Text(
                          "Gallery",
                          style: TextStyle(color: AppColors.hotelListDarkBlue),
                        ),
                        leading: Icon(
                          Icons.account_box,
                          color: AppColors.baseLightBlueColor,
                        ),
                      ),
                      Divider(
                        height: 1,
                        color: AppColors.baseLightBlueColor,
                      ),
                      ListTile(
                        onTap: () {
                          Navigator.pop(context, _pickImage(ImageSource.camera,context));
                        },
                        title: Text(
                          "Camera",
                          style: TextStyle(color: AppColors.hotelListDarkBlue,),
                        ),
                        leading: Icon(
                          Icons.camera,
                          color: AppColors.baseLightBlueColor,
                        ),
                      ),
                    ],
                  ),
                ),
              );
            });
      }
    
     ///ImageSource: Camera and Gallery.
      Future<Null> _pickImage(ImageSource source,context) async {
        final pickedImage = await ImagePicker().pickImage(source: source);
        _image = pickedImage != null ? File(pickedImage.path) : null;
        if (_image != null) {
          setState(() {
          });}}

////call this method 
GestureDetector(
                                  onTap: () {
_showChoiceDialog(context);},