Fix ItemGrid crash

[cewert]

Prevent crash by validating node ref. Comes from roku.com crash log:

   file/line: pkg:/components/ItemGrid/ItemGrid.brs(696) 
#0  Function swapdone() As Voi$1 Backtrace: 
'Dot' Operator attempted with invalid BrightScript Component or interface reference. (runtime error &hec) in pkg:/components/ItemGrid/ItemGrid.brs(696) 
m                roAssociativeArray refcnt=2 count:2 
global           Interface:ifGloba$1 Local Variables:

which points to this line after running build-prod on 2.1.2:

if m.swapAnimation.state = "stopped"

Issues

Ref #1164