[relations] No crash when clicking remove with no selection

Fix #9224

Author: m-kuhn

src/gui/qgsrelationmanagerdialog.cpp

@@ -118,6 +118,7 @@ void QgsRelationManagerDialog::on_mBtnAddRelation_clicked()
 
 void QgsRelationManagerDialog::on_mBtnRemoveRelation_clicked()
 {
+  if ( mRelationsTable->currentIndex().isValid() )
   mRelationsTable->removeRow( mRelationsTable->currentItem()->row() );
 }