fix: crashing if handler is missing

[sidhujus]

Which issue(s) does this change fix?

#6253

Why is this change necessary?

There is currently no validation on if the handler is defined within a template

How does it address the issue?

Checks if the handler is defined before trying to access and assign it

What side effects does this change have?

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jysheng123]

I would add a unit test raising the specific error thrown without the handler present other than that LGTM

[jysheng123]

Once the above comments are addressed it LGTM!

[lucashuy]

Thanks for addressing the blank string Handler, left a comment about testing

[lucashuy]

Thanks for addressing the changes