Tabs are lost after opening a private window and restarting the browser

[rhapsy]

When I open the browser with existing tabs and then open a private browsing window, if I close the main (non-private) browser window and reopen the browser, all previously opened tabs are gone.

Steps to reproduce

1. Open Zen Browser with several tabs already opened.
2. Open a private browsing window.
3. Close the main (non-private) browser window.
4. Reopen Zen Browser.

Expected behavior
The previously opened tabs should be restored.

Actual behavior
No tabs are restored; the browser opens with no tabs.