Fixes #1373: goForward and goBackward crash (#1378)
Fixes #1373: goForward and goBackward crash (#1378)

justindarc/focus,mozilla/focus,mozilla-mobile/focus-ios,justindarc/focus,mozilla-mobile/focus,mozilla-mobile/focus-ios,mozilla-mobile/focus,mozilla-mobile/focus,justindarc/focus,mozilla-mobile/focus,mozilla/focus,mozilla-mobile/focus,justindarc/focus,mozilla/focus,mozilla-mobile/focus-ios,mozilla-mobile/focus-ios,mozilla-mobile/focus-ios,mozilla/focus,mozilla-mobile/focus-ios,justindarc/focus,justindarc/focus,mozilla/focus,mozilla-mobile/focus