webapp is null on startup

[xabolcs]

STR:

0. get an addon with about: page (e.g. about:startup, about:telemetry, Nightly Tester Tools 3.4pre)
1. open it's about url (or simply open about:support from the Help menu)
2. open about:addons
3. arrange tabs to let about:addons be closer to the left
4. restart Application
5. see below:

    Error: webapp is null
    Source: chrome://webapptabs/content/webtab.js
    Line: 623

WebAppTabs is 3.0a1.