fix null crash and connection handling

[krushnarout]

Line of code, what is the cause that leads to this fix? Which variable is potentially null?

omi/app/lib/services/wals.dart

Line 287 in 94f16e1

var pd = await _device!.getDeviceInfo(connection);

omi/app/lib/services/wals.dart

Line 819 in 94f16e1

var pd = await _device!.getDeviceInfo(connection);

crash occurs because _device can become null after await, for example when the device disconnects, but the code still uses _device!

closes #3804

[gemini-code-assist]

Code Review

This pull request effectively resolves a critical null-pointer exception by adopting safer patterns for handling nullable class members in asynchronous methods. By caching the _device object in a local variable within SDCardWalSync and FlashPageWalSync, the code is now protected against race conditions where the device could be nulled after a check. Furthermore, the addition of null checks for the connection object returned by ensureConnection enhances the robustness of the connection handling logic. The changes are clean, correct, and consistently applied, representing a solid improvement to the codebase's stability.

[beastoin]

#3802 (comment)

[krushnarout]

#3802 (comment)

done #3805 (comment)

[beastoin]

lgtm @krushnarout