fix: second-instance additionalData may be null

[starpit]

Description of what you did:

This works around an issue where the additionalData parameter is null when using "popup window" mode. Since it is null, env vars, etc. from the second instance will not be transmitted to the first. But for now, we need to fix the bad behavior where one cannot even open a second window.

Fixes #9351

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Optional Items

• [ ] 💥 This PR involves a breaking change. If so, include "BREAKING CHANGE: ...why..." in the commit and PR message.

[k8s-ci-robot]

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