Why is the value in this map NULL? - flutter

Question

Error: "NoSuchMethodError: 'length' method not found. Receiver: null" when updating map values
List<ImageDetails> _images = [
ImageDetails(
imagePath: 'assets/images/meal1.jpg',
date: '2021-11-30',
details:
'',
),
ImageDetails(
imagePath: 'assets/images/meal2.jpg',
date: '2021-11-30',
details:
'',
),
];
var dateToImages = new Map();
_images.sort((a,b) => a.date.compareTo(b.date));
//group images by date
for (int i = 0; i < _images.length; i++) {
var d = _images[i].date; //convert string to Datetime
print("printing datetime in for loop");
print(d);
if (dateToImages.containsKey(d)) {
print("second element");
var list = dateToImages[d].add(_images[i]);
dateToImages[d] = list;
} else {
print("first element");
dateToImages[d] = [_images[i]];
}
}
var sortedKeys = dateToImages.keys.toList()..sort((a, b) => a.compareTo(b));
List<Widget> children = [];
print("=====printing datetoImages");
print(dateToImages);
print("======== printing sortedKeys");
print(sortedKeys);
int len = dateToImages['2021-11-30'].length;
Below is result of running above code
printing datetime in for loop
2021-11-30
first element
printing datetime in for loop
2021-11-30
second element
=====printing datetoImages
{2021-11-30: null}
======== printing sortedKeys
[2021-11-30]
After printing some variables, it seems like the issue is with the value for key "2021-11-30" in dateToImages being null... I don't understand why I keep getting null since it seems like the map building process in the for loop seems to be going well? Can anyone shed some light on this bug?
Thanks!

Answer 1

The error message suggests you are accessing length method which is not available.
since null doesn't have length method but list have it.
So, you may have a logical error, where you think you are returning list but the code is returning null
In your case:
Your Code is fine except for this part:
if (dateToImages.containsKey(d)) {
print("second element");
var list = dateToImages[d].add(_images[i]);
dateToImages[d] = list;
}
add() method returns void which will return null to var list and null will be sent to the map
Update it directly, replace above code with this:
if (dateToImages.containsKey(d)) {
print("second element");
dateToImages[d].add(_images[i]);
}

Answer 2

try this :
List<ImageDetails> _images = [
ImageDetails(
imagePath: 'assets/images/meal1.jpg',
date: '2021-12-30',
details:
'',
),
ImageDetails(
imagePath: 'assets/images/meal2.jpg',
date: '2021-11-30',
details:
'',
),
];
var dateToImages = new Map();
_images.forEach((img){
if(dateToImages.containsKey(img.date)){
dateToImages[img.date].add(img);
}else{
dateToImages[img.date] = [img];
}
});
var sortedKeys = dateToImages.keys.toList()..sort((a, b) =>
a.compareTo(b));
print("=====printing datetoImages");
print(dateToImages);
print("======== printing sortedKeys");
print(sortedKeys);
int len = dateToImages['2021-11-30'].length;
Output:
=====printing datetoImages
{2021-12-30: [Instance of 'ImageDetails'], 2021-11-30: [Instance of
'ImageDetails']}
======== printing sortedKeys
[2021-11-30, 2021-12-30]