Fix thread snapshot UI due to variable initialization

[kvbp2kfb]

== RELEASE NOTES ==

General Changes
* The worker page's thread snapshot UI does not work (no stack trace displayed on click) when there is active query load (tested under Chrome). This patch fixes an uninitialized variable in client JS that was causing this UI behavior.

[swapsmagic]

Seems like some ui tests failing, let's fix them before we can merge it.

[caithagoras]

@kvbp2kfb The entire release notes section should be enclosed with a pair of triple quotes, not just the single line. I have fixed that for you. You can click on "... -> edit" to see what proper format so that you would know for the next time. Thanks!

[caithagoras]

Release notes should be user-facing. What does user see before this fix? Was the page failing to load or something?