adding defensive check for existence of select2 on the target element during the document mousedown/touchend events
Merge pull request #764 from socialceramics/postdestroy-dropdown-error

Adding defensive check during the document mousedown/touchend event handler.
Fix issue introduced by 20950e0f0cbbaa2ed9ddb3306320a69d4691c87d.
Selected items and other invisible items were not ignored by highlightUnderEvent when calculating index.
This caused highlighting of incorrect items when the list contained hidden items.
Merge pull request #768 from bcoughlan/master

Fix issue introduced by 20950e0.
do not close the dropdown on resize or scroll, instead keep it positioned. fixes #753 #669 #719
fixes #770
remove dead code. fixes #772
dont error out when 'x' is removed really fast multiple times. fixes #745