Check canvas is not nil before releasing it.
Check canvas is not nil before releasing it.

CodyGuo/gxui,henrikhodne/gxui,abadojack/gxui,google/gxui,Drahoslav7/gxui,pombredanne/gxui,rnubel/gxui,treejames/gxui,ylong/gxui,henrylee2cn/gxui,Anaminus/gxui,UIKit0/gxui,jmptrader/gxui,leelib/gxui,crr0004/gxui,MeGaPk/gxui,insionng/gxui,mzgnr/gxui,cainiaocome/gxui,xinhuang327/gxui