KRearrangeColumnsProxyModel: fix assert in index(0, 0) on empty model

REVIEW: 128578

autotests/krearrangecolumnsproxymodeltest.cpp

@@ -78,6 +78,23 @@ private Q_SLOTS:
         QCOMPARE(pm.columnCount(), 0);
     }
 
+    void shouldShowNothingIfNoRows()
+    {
+        // Given a rearrange-columns proxy
+        KRearrangeColumnsProxyModel pm;
+        pm.setSourceColumns(QVector<int>() << 2 << 3 << 1 << 0);
+
+        // When using that proxy on top of an empty source model
+        QStandardItemModel sourceModel;
+        sourceModel.setColumnCount(4);
+        pm.setSourceModel(&sourceModel);
+
+        // Then the proxy should show nothing
+        QCOMPARE(pm.rowCount(), 0);
+        QCOMPARE(pm.columnCount(), 4);
+        QCOMPARE(pm.index(0, 0), pm.index(0, 0)); // like QAbstractItemView::setModel does in a Q_ASSERT_X
+    }
+
     void shouldRearrangeColumns()
     {
         // Given a rearrange-columns proxy

src/krearrangecolumnsproxymodel.cpp

@@ -69,7 +69,9 @@ QModelIndex KRearrangeColumnsProxyModel::index(int row, int column, const QModel
 
     // Find the child in the source model, we need its internal pointer
     const QModelIndex sourceIndex = sourceModel()->index(row, sourceColumnForProxyColumn(column), sourceParent);
-    Q_ASSERT(sourceIndex.isValid());
+    if (!sourceIndex.isValid()) {
+        return QModelIndex();
+    }
 
     return createIndex(row, column, sourceIndex.internalPointer());
 }