To reproduce:
- Create a PropertyGrid and initialize it like this:
PropertyStoreItem checkboxItem = new PropertyStoreItem(typeof(bool), "Checkbox", true);
_store = new RadPropertyStore();
_store.Add(checkboxItem);
this.radPropertyGrid1.SelectedObject = _store;

-change the store item first to null then to false

Workaround:
-Before you set the item value to false set it to true first like this:
item.Value = null;

//set first to true then to false
item.Value = true;
item.Value = false;