fix: add null checks for frm.timeline in activity.js

[trufurs]

Description

This PR fixes the form bug when the form is opened for a virtual doctype.

Relevant Technical Choices

Testing Instructions

Additional Information:

Screenshot/Screencast

Checklist

• I have carefully reviewed the code before submitting it for review.
• This code is adequately covered by unit tests to validate its functionality.
• I have conducted thorough testing to ensure it functions as intended.
• A member of the QA team has reviewed and tested this PR (To be checked by QA or code reviewer)

Fixes #55