[expression] autoselect first item in function list

(cherry-picked from 395d630)

Author: slarosa

src/gui/qgsexpressionbuilderwidget.cpp

@@ -81,6 +81,11 @@ QgsExpressionBuilderWidget::QgsExpressionBuilderWidget( QWidget *parent )
   {
     tab_2->setEnabled( false );
   }
+
+  // select the first item in the function list 
+  // in order to avoid a blank help widget
+  QModelIndex firstItem = mProxyModel->index( 0, 0, QModelIndex() );
+  expressionTree->setCurrentIndex( firstItem );
 }