projectlistview.cpp: check for null pointer

Fixes Coverity CID 709291: Explicit null dereferenced (FORWARD_NULL)
Passing null variable "item" to function "FolderProjectItem::groupName() const", which dereferences it. [show details]
272                        props.insert("groupname", item->groupName());
...
Assigning: "item" = 0.
276                } else item = NULL;