WIP: fix missing null check that blocks multi-tab copy-paste

[marcellobarile]

Proposed Changes

⚠️ Adds a condition to avoid breaking the library during a multi-tab copy-paste

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[nikku]

@marcellobarile Happy to merge this if we can reproduce the issue in a real world scenario, and a test case is attached.

[marcellobarile]

Sure this is still in draft, I'll get back at it and the multi-tab copy/paste topic once capacity allows