Fix error when plugin don't extend book
Fix error when plugin don't extend book

codepiano/gitbook,youprofit/gitbook,gdbooks/gitbook,anrim/gitbook,gaearon/gitbook,JozoVilcek/gitbook,palerdot/gitbook,guiquanz/gitbook,iflyup/gitbook,vehar/gitbook,gaearon/gitbook,npracht/documentation,sudobashme/gitbook,a-moses/gitbook,shibe97/gitbook,xxxhycl2010/gitbook,rohan07/gitbook,FKV587/gitbook,xxxhycl2010/gitbook,mruse/gitbook,GitbookIO/gitbook,CN-Sean/gitbook,escopecz/documentation,intfrr/gitbook,sunlianghua/gitbook,nycitt/gitbook,switchspan/gitbook,tzq668766/gitbook,intfrr/gitbook,shibe97/gitbook,hujianfei1989/gitbook,FKV587/gitbook,ferrior30/gitbook,jasonslyvia/gitbook,sudobashme/gitbook,justinleoye/gitbook,wewelove/gitbook,youprofit/gitbook,Abhikos/gitbook,sunlianghua/gitbook,2390183798/gitbook,palerdot/gitbook,haamop/documentation,xcv58/gitbook,xiongjungit/gitbook,mruse/gitbook,mautic/documentation,bjlxj2008/gitbook,ryanswanson/gitbook,jocr1627/gitbook,gencer/gitbook,boyXiong/gitbook,snowsnail/gitbook,guiquanz/gitbook,tshoper/gitbook,megumiteam/documentation,JohnTroony/gitbook,xcv58/gitbook,gdbooks/gitbook,2390183798/gitbook,minghe/gitbook,bjlxj2008/gitbook,hongbinz/gitbook,a-moses/gitbook,strawluffy/gitbook,iamchenxin/gitbook,bradparks/gitbook,bradparks/gitbook,JohnTroony/gitbook,kamyu104/gitbook,nycitt/gitbook,athiruban/gitbook,mautic/documentation,webwlsong/gitbook,hujianfei1989/gitbook,ferrior30/gitbook,athiruban/gitbook,qingying5810/gitbook,wewelove/gitbook,kamyu104/gitbook,alex-dixon/gitbook,yaonphy/SwiftBlog,qingying5810/gitbook,jasonslyvia/gitbook,OriPekelman/gitbook,ZachLamb/gitbook,justinleoye/gitbook,OriPekelman/gitbook,escopecz/documentation,tshoper/gitbook,megumiteam/documentation,lucciano/gitbook,thelastmile/gitbook,hongbinz/gitbook,boyXiong/gitbook,Keystion/gitbook,rohan07/gitbook,tzq668766/gitbook,jocr1627/gitbook,haamop/documentation,webwlsong/gitbook,codepiano/gitbook,grokcoder/gitbook,grokcoder/gitbook,switchspan/gitbook,gencer/gitbook,snowsnail/gitbook,vehar/gitbook,thelastmile/gitbook,alex-dixon/gitbook,iamchenxin/gitbook,lucciano/gitbook,CN-Sean/gitbook,minghe/gitbook,iflyup/gitbook,ZachLamb/gitbook,npracht/documentation,xiongjungit/gitbook