[Content-Perspective] Closing TAB of frozen node causes ClassCastException

Reproduction:

• Open a document
• Associate with project Q
• Click Edit, make some changes, and click "Save"
• Checkout Core (you'll see jcr:frozenNode in the TAB title)
• Click the Close icon of the TAB
  Expect: TAB is closed
  Actual: java.lang.ClassCastException: org.hippoecm.repository.jackrabbit.XASessionImpl cannot be cast to org.hippoecm.repository.api.HippoSession
  at org.hippoecm.frontend.editor.HippostdPublishableEditor.isModified(HippostdPublishableEditor.java:375)