PHP Fatal error: Uncaught TypeError: Argument 1 passed to WP_Rocket\Dependencies\Minify\JS when update WPR while RUCSS is in progress

[Mai-Saad]

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Describe the bug
Fatal error added to the log file when upgrading plugin while having background process in progress

To Reproduce
Steps to reproduce the behavior:

1. Using version 3.9.4, Enable RUCSS
2. On the plugins page, update WPR to 3.9.4.1 or latest (While RUCSS is still in progress)
3. Check the debug.log

Expected behavior
No Error in debug.log file

Screenshots
If applicable, add screenshots to help explain your problem.
debug.log

Additional context

• Error is not affecting using of the dashboard nor the test site
• We need to check while CPCSS / preload is in progress too
• If we switched between branches in the same scenario without reactivate plugin, we will see different log error

Backlog Grooming (for WP Media dev team use only)

• Reproduce the problem
• Identify the root cause
• Scope a solution
• Estimate the effort

[piotrbak]

Other possible error when reproducing the issue:
debug