Added QAbstractItemModelTester to main and proxy model. And fixing errors in functions flags(...), rowCount(...) and columnCount(...).

model/tablemodel.cpp

@@ -39,18 +39,25 @@ TableModel::TableModel(QUndoStack* undoStack, QObject* parent)
     , m_undoStack(undoStack) {}
 
 Qt::ItemFlags TableModel::flags(const QModelIndex& index) const {
+  if (!index.isValid()) {
+    return QAbstractTableModel::flags(index);
+  }
   return Qt::ItemIsEditable | QAbstractTableModel::flags(index);
 }
 
 QHash<int, QByteArray> TableModel::roleNames() const { return ROLE_NAMES; }
 
 int TableModel::rowCount(const QModelIndex& parent) const {
-  Q_UNUSED(parent);
+  if (parent.isValid()) {
+    return 0;  // no children
+  }
   return m_items.size();
 }
 
 int TableModel::columnCount(const QModelIndex& parent) const {
-  Q_UNUSED(parent);
+  if (parent.isValid()) {
+    return 0;  // no children
+  }
   return ROLE_NAMES.size();
 }