FIX: Edge case attempting to destroy uninitialized tabs

Another edge case very similar to
https://github.com/silverstripe/sapphire/commit/07fb7563273fa82bd93eb255
70571c0a298198e5
Fix: RequestHandler not managing 404/403 errors correctly (fixes #1696)
Merge pull request #1741 from kinglozzer/1696_handlerequest_fix

Fix: RequestHandler not managing 404/403 errors correctly (fixes #1696)
Update configuration.md

Link to ssbits.com config cheatsheet no longer relevant.
Merge pull request #1744 from ryanwachtl/patch-1

Update configuration.md
Merge pull request #1739 from kinglozzer/leftandmain-uninitialized-tabs

FIX: Edge case attempting to destroy uninitialized tabs
BUG Fixed broken ErrorPage icons in CMS view
Merge pull request #1746 from tractorcow/3.1-errorpage-icons-fix

BUG Fixed broken ErrorPage icons in CMS view
BUG: Added setExcludedID() to exclude a certain record from uniqueness checking.
NEW: Implemented setList and setListProvider to provide DataList
Merge branch '1399-ajaxutf-excludeid' of git://github.com/svandragt/sapphire into 1399-ajaxutf-excludeid

Conflicts:
	forms/AjaxUniqueTextField.php