Fix missing module source panic

[cam72cam]

Crash should instead be a diags error

Resolves #1887

Target Release

1.9.0, 1.8.1

Checklist

• I have read the contribution guide.
• I have not used an AI coding assistant to create this PR.
• I have written all code in this PR myself OR I have marked all code I have not written myself (including modified code, e.g. copied from other places and then modified) with a comment indicating where it came from.

Go checklist

• I have run golangci-lint on my change and receive no errors relevant to my code.
• I have run existing tests to ensure my code doesn't break anything.
• I have added tests for all relevant use cases of my code, and those tests are passing.
• I have only exported functions, variables and structs that should be used from other packages.
• I have added meaningful comments to all exported functions, variables, and structs.

Website/documentation checklist

• I have locally started the website as described here and checked my changes.

[github-actions]

Reminder for the PR assignee: If this is a user-visible change, please update the changelog as part of the PR.