about:tor page is not loading in pre-nightly Tor Browser builds based on ESR60

Testing a pre-nightly Tor Browser build based on ESR60 it turns out the about:tor page is not loading. In the error console exceptions are thrown:

[Exception... "Component returned failure code: 0x80004004 (NS_ERROR_ABORT) [nsIPrefBranch.getComplexValue]"  nsresult: "0x80004004 (NS_ERROR_ABORT)"  location: "JS frame :: jar:file:///path/to/tor-browser/tor-browser_en-US/Browser/browser/omni.ja!/components/nsBrowserContentHandler.js :: get startPage :: line 660"  data: no]

I guess, not treating that browser.startup.homepage as preference with a complex value is not working as well as we thought.

(However, it worked while I was testing the patch for legacy/trac#26100 (moved) for some reason).