fix provider crash with mounted checks

[krushnarout]

moved provider checks outside of async callbacks, and added mounted checks after awaits and before Navigator.pop(context)

closes #3431

[gemini-code-assist]

Code Review

This pull request addresses potential crashes by adding mounted checks after await calls and before using BuildContext, which is a great improvement. It also refactors provider access to be outside of asynchronous callbacks, making the code safer. I've identified two critical issues where firstWhere is used to find a device, which could lead to a crash if the device is not found. I've suggested using firstWhereOrNull to handle these cases gracefully.