[FIX] ux3/NavigationBar: Make sure arrow is available when accessing

When several rerenderings happen close to each other, the 
_checkOverflow method might be called with a DOM-reference that does
not exist in the document anymore. Now the method checks for the current
DOM-Reference as well and does not do anything if there is no current 
DOM.

Change-Id: I1187f89df497fd2c7164fcd20c1ee14be10ce692
Merge "[FIX] ux3/NavigationBar: Make sure arrow is available when accessing"