wallpapers not appearing in desktop settings

[Ruchir]

After recent upgrade which added more new wallpapers, all wallpapers don't appear at once in selection. 3-5 wallpapers appear when I open desktop settings. When I click empty area then new wallpapers appear in that area. Before and after click screenshots attached.

[Ruchir (ruchir-brahmbhatt)]

Additional wallpapers after clicking empty area.

[George Sofianos (georgesofianosgr)]

For me the bug started when I choose Pictures category.
After that when I choose a wallpaper all the following wallpapers disappeared for the grid (in any cateogy).
If I click again the same wallpaper they appear again.

[George Sofianos (georgesofianosgr)]

Reproduce:
  * set an icon theme that doesn't include selection-checked icon (such as paper icon theme)
  * choose a wallpaper in the list.

The problem is that when wallpaper container draws itself it won't check for null icon.
My branch fixes this problem.