Fix application hang when closing window.
Fix application hang when closing window.

nyimbi/tilemill,Zhao-Qi/tilemill,nyimbi/tilemill,mbrukman/tilemill,Zhao-Qi/tilemill,paulovieira/tilemill-clima,Zhao-Qi/tilemill,tilemill-project/tilemill,MappingKat/tilemill,tizzybec/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,tizzybec/tilemill,florianf/tileoven,MappingKat/tilemill,MappingKat/tilemill,florianf/tileoven,Zhao-Qi/tilemill,paulovieira/tilemill-clima,nyimbi/tilemill,mbrukman/tilemill,Zhao-Qi/tilemill,MappingKat/tilemill,paulovieira/tilemill-clima,fxtentacle/tilemill,tizzybec/tilemill,fxtentacle/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,fxtentacle/tilemill,isaacs/tilemill,MappingKat/tilemill,fxtentacle/tilemill,nyimbi/tilemill,isaacs/tilemill,fxtentacle/tilemill,tizzybec/tilemill,tilemill-project/tilemill,mbrukman/tilemill,paulovieira/tilemill-clima,mbrukman/tilemill,florianf/tileoven,tilemill-project/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,isaacs/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,tizzybec/tilemill,nyimbi/tilemill,mbrukman/tilemill,paulovieira/tilemill-clima