fid is the default if the ID column is empty when we create a geopackage

src/gui/qgsnewgeopackagelayerdialog.cpp

@@ -44,6 +44,8 @@
 #include <cpl_error.h>
 #include <cpl_string.h>
 
+#define DEFAULT_OGR_FID_COLUMN_TITLE "fid" // default value from OGR
+
 QgsNewGeoPackageLayerDialog::QgsNewGeoPackageLayerDialog( QWidget *parent, Qt::WindowFlags fl )
   : QDialog( parent, fl )
 {
@@ -85,7 +87,7 @@ QgsNewGeoPackageLayerDialog::QgsNewGeoPackageLayerDialog( QWidget *parent, Qt::W
   mGeometryWithMCheckBox->setEnabled( false );
   mGeometryColumnEdit->setEnabled( false );
   mGeometryColumnEdit->setText( "geometry" );
-  mFeatureIdColumnEdit->setText( "fid" );
+  mFeatureIdColumnEdit->setPlaceholderText( DEFAULT_OGR_FID_COLUMN_TITLE );
   mCheckBoxCreateSpatialIndex->setEnabled( false );
   mCrsSelector->setEnabled( false );
 
@@ -192,7 +194,8 @@ void QgsNewGeoPackageLayerDialog::mAddAttributeButton_clicked()
   if ( !mFieldNameEdit->text().isEmpty() )
   {
     QString myName = mFieldNameEdit->text();
-    if ( myName == mFeatureIdColumnEdit->text() )
+    const QString featureId = mFeatureIdColumnEdit->text().isEmpty() ? DEFAULT_OGR_FID_COLUMN_TITLE : mFeatureIdColumnEdit->text();
+    if ( myName.compare( featureId, Qt::CaseInsensitive ) == 0 )
     {
       QMessageBox::critical( this, tr( "Add Field" ), tr( "The field cannot have the same name as the feature identifier." ) );
       return;