NullPointerException when closing revision TAB of document

Reproduction:

In the Channel editor

• Open channel
• Edit master version of a document and save
• Switch to a project
• Click edit button of the same document
• Click "Add to project". Don't save but close the document (publish button is active, known issue CHANNELMGR-1973)
• Click "Open in content editor"
• Click "Document" -> "Show revision history ..." and click a revision to open a TAB
• Close the revision TAB

Expected: TAB closed and editor active
Actual: 

Root cause:

java.lang.NullPointerException
     at org.hippoecm.frontend.editor.HippostdPublishableEditor.close(HippostdPublishableEditor.java:586)