[partition] Avoid crash if combobox is gone and model resets

- Since the lambda is connected to the model only, it should
   check that the target combo exists as well.

src/modules/partition/gui/ChoicePage.cpp

@@ -1027,6 +1027,7 @@ ChoicePage::updateActionChoicePreview( ChoicePage::InstallChoice choice )
                 connect( m_core->bootLoaderModel(), &QAbstractItemModel::modelReset,
                     [ this ]()
                     {
+                        if ( !m_bootloaderComboBox.isNull() )
                             Calamares::restoreSelectedBootLoader( *m_bootloaderComboBox, m_core->bootLoaderInstallPath() );
                     }
                 );