fix #3690: NPE while exporting GPX
fix #3690: NPE while exporting GPX

brok85/cgeo,kumy/cgeo,Bananeweizen/cgeo,madankb/cgeo,rsudev/c-geo-opensource,S-Bartfast/cgeo,Bananeweizen/cgeo,superspindel/cgeo,matej116/cgeo,xiaoyanit/cgeo,KublaikhanGeek/cgeo,marco-dev/c-geo-opensource,auricgoldfinger/cgeo,Bananeweizen/cgeo,mucek4/cgeo,samueltardieu/cgeo,matej116/cgeo,schwabe/cgeo,yummy222/cgeo,kumy/cgeo,ThibaultR/cgeo,pstorch/cgeo,madankb/cgeo,brok85/cgeo,samueltardieu/cgeo,mucek4/cgeo,cgeo/cgeo,madankb/cgeo,SammysHP/cgeo,SammysHP/cgeo,KublaikhanGeek/cgeo,tobiasge/cgeo,KublaikhanGeek/cgeo,schwabe/cgeo,superspindel/cgeo,S-Bartfast/cgeo,marco-dev/c-geo-opensource,rsudev/c-geo-opensource,ThibaultR/cgeo,S-Bartfast/cgeo,auricgoldfinger/cgeo,yummy222/cgeo,pstorch/cgeo,tobiasge/cgeo,cgeo/cgeo,marco-dev/c-geo-opensource,lewurm/cgeo,ThibaultR/cgeo,vishwakulkarni/cgeo,cgeo/cgeo,Huertix/cgeo,Huertix/cgeo,mucek4/cgeo,pstorch/cgeo,auricgoldfinger/cgeo,SammysHP/cgeo,vishwakulkarni/cgeo,samueltardieu/cgeo,schwabe/cgeo,cgeo/cgeo,Huertix/cgeo,superspindel/cgeo,lewurm/cgeo,matej116/cgeo,xiaoyanit/cgeo,tobiasge/cgeo,schwabe/cgeo,xiaoyanit/cgeo,yummy222/cgeo,kumy/cgeo,lewurm/cgeo,rsudev/c-geo-opensource,brok85/cgeo,vishwakulkarni/cgeo