Fix crash with wxDataViewVirtualListModel in generic wxDataViewCtrl

Recent sorting-related changes resulted in calling FindNode(), which
can only be used with the non-"virtual list" models, unconditionally,
after the items values was changed by user, resulting in a crash.

Add the missing IsVirtualList() check and also add a comment explicitly
stating that all code involving wxDataViewTreeNode can only be used
after checking that IsVirtualList() returns false.

Closes #18057.

src/generic/datavgen.cpp

@@ -458,6 +458,11 @@ class wxDataViewTreeNode;
 
 typedef wxVector<wxDataViewTreeNode*> wxDataViewTreeNodes;
 
+// Note: this class is not used at all for virtual list models, so all code
+// using it, i.e. any functions taking or returning objects of this type,
+// including wxDataViewMainWindow::m_root, can only be called after checking
+// that we're using a non-"virtual list" model.
+
 class wxDataViewTreeNode
 {
 public:
@@ -2941,6 +2946,8 @@ bool wxDataViewMainWindow::ItemDeleted(const wxDataViewItem& parent,
 }
 
 bool wxDataViewMainWindow::DoItemChanged(const wxDataViewItem & item, int view_column)
+{
+    if ( !IsVirtualList() )
     {
         // Move this node to its new correct place after it was updated.
         //
@@ -2953,6 +2960,7 @@ bool wxDataViewMainWindow::DoItemChanged(const wxDataViewItem & item, int view_c
         wxDataViewTreeNode* const node = FindNode(item);
         wxCHECK_MSG( node, false, "invalid item" );
         node->PutInSortOrder(this);
+    }
 
     wxDataViewColumn* column;
     if ( view_column == wxNOT_FOUND )