Fixes #11545 - Fix the null safety issue in shimmer example in cookbook

[kishan-dhankecha]

Issues fixed by this PR (if any): #11545

I am not sure if I need to make changes in all the files that I changed or not so I'll keep this change in the Draft until someone corrects me If I made the wrong changes.

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[kishan-dhankecha]

Note that this issue of null check only happens when a user forgets to wrap the widget with Shimmer. In that case, context will not find the Shimmer on the upper side of the tree.

[sfshaza2]

This isn't correct. Since the "shimmer" variable hasn't been created with a question mark, it cannot be null, so you don't have to test it for null. For more info, see:
https://dart.dev/null-safety/understanding-null-safety

[sfshaza2]

Closing.

[kishan-dhankecha]

Note that this issue of null check only happens when a user forgets to wrap the widget with Shimmer. In that case, context will not find the Shimmer on the upper side of the tree.

As said above this issue happens only when the Shimmer widget is not added to the parent stack. For this to work first we have to add Shimmer widget at the top of the widget tree. (Ideally near the MaterialApp). After that to show the actual animation for any widget we will wrap that with ShimmerLoading widget. If somehow the user does not provide the Shimmer widget at the top this thorws a null error.