Nullptr safety.

(cherry picked from commit 071bdbdaa7)

common/rc_item.cpp

@@ -406,6 +406,9 @@ void RC_TREE_MODEL::GetValue( wxVariant&              aVariant,
                               wxDataViewItem const&   aItem,
                               unsigned int            aCol ) const
 {
+    if( !aItem.IsOk() )
+        return;
+
     const RC_TREE_NODE*            node = ToNode( aItem );
     const std::shared_ptr<RC_ITEM> rcItem = node->m_RcItem;
     MARKER_BASE*                   marker = rcItem->GetParent();
@@ -478,8 +481,10 @@ bool RC_TREE_MODEL::GetAttr( wxDataViewItem const&   aItem,
                              unsigned int            aCol,
                              wxDataViewItemAttr&     aAttr ) const
 {
+    if( !aItem.IsOk() )
+        return false;
+
     const RC_TREE_NODE* node = ToNode( aItem );
-    wxASSERT( node );
 
     bool ret = false;
     bool heading = node->m_Type == RC_TREE_NODE::MARKER;