be null-safe when checking `document.body.tagName`
be null-safe when checking `document.body.tagName`

fixes #1365

dimatter/vimium,elderbas/vimium,elegantwww/vimium,elegantwww/vimium,dimatter/vimium,philc/vimium,poacher2k/vimium,csmalin/vimium,crazypenguincode/vimium,PickRelated/vimium,Lw-Cui/vimium,tuchangwei/vimium,willsALMANJ/vimium,borgified/vimium,hhsue/vimium,dimatter/vimium,wildeyes/vimium,decaffeinate-examples/vimium,smblott-github/vimium,Aleroniponi/vimium,zhangmin510/vimium,decaffeinate-examples/vimium,justalittlenoob/vimium,elderbas/vimium,justalittlenoob/vimium,Lw-Cui/vimium,mrmr1993/vimium,PrestanceDesign/vimium,Robinson10240/vimium,Aleroniponi/vimium,gdh1995/vimium,wildeyes/vimium,VPashkov/vimium,zhangmin510/vimium,cyrixhero/vimium,hhsue/vimium,PrestanceDesign/vimium,PickRelated/vimium,tuchangwei/vimium,PickRelated/vimium,Robinson10240/vimium,crazypenguincode/vimium,douglas-larocca/vimium,smblott-github/vimium,cyrixhero/vimium,borgified/vimium,Robinson10240/vimium,borgified/vimium,elegantwww/vimium,poacher2k/vimium,douglas-larocca/vimium,bkudria/vimium,decaffeinate-examples/vimium,douglas-larocca/vimium,PrestanceDesign/vimium,smblott-github/vimium,VPashkov/vimium,mrmr1993/vimium,mrmr1993/vimium,hhsue/vimium,Aleroniponi/vimium,tuchangwei/vimium,csmalin/vimium,philc/vimium,gdh1995/vimium,gdh1995/vimium,willsALMANJ/vimium,wildeyes/vimium,csmalin/vimium,philc/vimium,willsALMANJ/vimium,VPashkov/vimium,bkudria/vimium,justalittlenoob/vimium,bkudria/vimium,zhangmin510/vimium,cyrixhero/vimium,elderbas/vimium,poacher2k/vimium,Lw-Cui/vimium