Let the ListModel wizard generate a valid model.

share/qtcreator/templates/wizards/listmodel/listmodel.cpp

@@ -17,7 +17,7 @@ int %ClassName%::rowCount(const QModelIndex &) const
     return items.size();
 }
 
-QVariant %ClassName%::data(const QModelIndex &index, int)
+QVariant %ClassName%::data(const QModelIndex &index, int) const
 {
     return items.at(index.row());
 }

share/qtcreator/templates/wizards/listmodel/listmodel.h

@@ -11,7 +11,7 @@ public:
     void addItems(const QList<%Datatype%> &items);
 
     virtual int rowCount(const QModelIndex &parent = QModelIndex()) const;
-    virtual QVariant data(const QModelIndex &index, int role = Qt::DisplayRole);
+    virtual QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const;
 
 private:
     QList<%Datatype%> items;