Fix crash in merge features dialog

Fixes #19119

Author: nyalldawson

src/app/qgsmergeattributesdialog.cpp

@@ -361,10 +361,8 @@ void QgsMergeAttributesDialog::refreshMergedValue( int col )
   mUpdating = false;
 }
 
-QVariant QgsMergeAttributesDialog::featureAttribute( QgsFeatureId featureId, int col )
+QVariant QgsMergeAttributesDialog::featureAttribute( QgsFeatureId featureId, int fieldIdx )
 {
-  int fieldIdx = mTableWidget->horizontalHeaderItem( col )->data( FieldIndex ).toInt();
-
   int i;
   for ( i = 0; i < mFeatureList.size() && mFeatureList.at( i ).id() != featureId; i++ )
     ;

src/app/qgsmergeattributesdialog.h

@@ -81,7 +81,7 @@ class APP_EXPORT QgsMergeAttributesDialog: public QDialog, private Ui::QgsMergeA
     //! Calculates the merged value of a column (depending on the selected merge behavior) and inserts the value in the corresponding cell
     void refreshMergedValue( int col );
     //! Inserts the attribute value of a specific feature into the row of merged attributes
-    QVariant featureAttribute( QgsFeatureId featureId, int col );
+    QVariant featureAttribute( QgsFeatureId featureId, int fieldIdx );
     //! Appends the values of the features for the final value
     QVariant concatenationAttribute( int col );