Better handling of non-existent slides in TrainingModulesUsers
Better handling of non-existent slides in TrainingModulesUsers

Resolves #1075

sejalkhatri/WikiEduDashboard,sejalkhatri/WikiEduDashboard,Wowu/WikiEduDashboard,KarmaHater/WikiEduDashboard,alpha721/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,feelfreelinux/WikiEduDashboard,majakomel/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,sejalkhatri/WikiEduDashboard,Wowu/WikiEduDashboard,majakomel/WikiEduDashboard,KarmaHater/WikiEduDashboard,feelfreelinux/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,feelfreelinux/WikiEduDashboard,KarmaHater/WikiEduDashboard,majakomel/WikiEduDashboard,Wowu/WikiEduDashboard,Wowu/WikiEduDashboard,alpha721/WikiEduDashboard,KarmaHater/WikiEduDashboard,sejalkhatri/WikiEduDashboard,alpha721/WikiEduDashboard,alpha721/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,sejalkhatri/WikiEduDashboard,feelfreelinux/WikiEduDashboard,majakomel/WikiEduDashboard