avoid javascript error if user saves without providing a datasource path
avoid javascript error if user saves without providing a datasource path

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