Get mFeature before updating attributeform

The issue was, that the attributeform didn't got updated in case there was the first entry in the combobox selected.
If there was not the first combobox entry selected, there has been a setindex after the initialization and the attributeform got updated.
But the first entry needs this call.

Fix #19342

src/gui/editorwidgets/qgsrelationreferencewidget.cpp

@@ -555,6 +555,7 @@ void QgsRelationReferenceWidget::init()
 
     // Only connect after iterating, to have only one iterator on the referenced table at once
     connect( mComboBox, static_cast<void ( QComboBox::* )( int )>( &QComboBox::currentIndexChanged ), this, &QgsRelationReferenceWidget::comboReferenceChanged );
+    mReferencedLayer->getFeatures( mComboBox->currentFeatureRequest() ).nextFeature( mFeature );
     updateAttributeEditorFrame( mFeature );
     QApplication::restoreOverrideCursor();
   }