Fixes bug crash when setting the cover image to nil.
Fixes bug crash when setting the cover image to nil.

shads196770/tapkulibrary,joystate/CustomizebleCalendar,triage/tapkulibrary,BartonJT/tapkulibrary,joystate/CustomizebleCalendar,pengleelove/tapkulibrary,adan016/tapkulibrary,bpap1989/test,venkadesh02/tapkulibrary,yybutcher/tapkulibrary,joystate/CustomizebleCalendar,coresuite/tapkulibrary,weiyanwu84/tapkulibrary,mohsinalimat/tapkulibrary,drchrono/tapkulibrary,joystate/CustomizebleCalendar