Mitigate two potential nil pointer dereference issues.

[x676f64]

Proposed changes

These changes patch two cases where a nil pointer dereference can occur.

Checklist

• [X ] Pull request is created against the dev branch
• All checks passed (lint, unit/integration/regression tests etc.) with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[GeorginaReeder]

Thanks so much for your contribution @x676f64 , we appreciate it! :)

[tarunKoyalwar]

@x676f64 , thanks for contributing but there doesn't seem to be any nil pointer dereference errors or a chance of it in based on changes

In golang Nil Pointer dereference only occurs when pointers are used and that is not the case in StackMonitor and for defer unlock in callback , mutex was used at that place because the variable was updated , invoking callbacks doesn't require syncronization

closing this pr for now , feel free to create new pr if you encountered a panic

[tarunKoyalwar]

closing since there doesn't seem to be issue in code