contacts: fix crash when deselecting contacts

- fixes crash when a contact item is deselected

Change-Id: I41801da8b62ff91f477bda1e6986d7c19255b462
Tuleap: #1024

SmartPanel.xaml.cpp

@@ -228,9 +228,14 @@ SmartPanel::_smartList__SelectionChanged(Platform::Object^ sender, Windows::UI::
 {
     auto listbox = safe_cast<ListBox^>(sender);
     auto item = safe_cast<SmartPanelItem^>(listbox->SelectedItem);
+    if (item != nullptr) {
         auto contact = safe_cast<Contact^>(item->_contact);
         ContactsViewModel::instance->selectedContact = contact;
     }
+    else {
+        ContactsViewModel::instance->selectedContact = nullptr;
+    }
+}
 
 void
 SmartPanel::_accountList__SelectionChanged(Platform::Object^ sender, Windows::UI::Xaml::Controls::SelectionChangedEventArgs^ e)