fix #4450: crash after performing backup
fix #4450: crash after performing backup

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