Header Data Oopsie (#181)

If the section or role is not supported, return an invalid QVariant, not an invalid QModelIndex. This fixes all of the headers across all of the editor views and makes them visible again.

Models/RepeatedModel.h

@@ -72,7 +72,7 @@ class RepeatedModel : public ProtoModel {
     auto data = ProtoModel::headerData(section, orientation, role);
     if (data.isValid()) return data;
     if (section <= 0 || role != Qt::DisplayRole || orientation != Qt::Orientation::Horizontal)
-      return QModelIndex();
+      return QVariant(); // << invalid
     return QString::fromStdString(_field->message_type()->field(section - 1)->name());
   }