[themes] Handle non-existent theme name setting in app stylesheet

Author: nirvn

src/app/qgisappstylesheet.cpp

@@ -124,7 +124,7 @@ void QgisAppStyleSheet::buildStyleSheet( const QMap<QString, QVariant> &opts )
   ss += QLatin1String( "QGroupBox{ font-weight: 600; }" );
 
   QString themeName = settings.value( QStringLiteral( "UI/UITheme" ), "default" ).toString();
-  if ( themeName == QStringLiteral( "default" ) )
+  if ( themeName == QStringLiteral( "default" ) || !QgsApplication::uiThemes().contains( themeName ) )
   {
     //sidebar style
     QString style = "QListWidget#mOptionsListWidget {"

src/app/qgsoptions.cpp

@@ -567,6 +567,10 @@ QgsOptions::QgsOptions( QWidget *parent, Qt::WindowFlags fl, const QList<QgsOpti
   whileBlocking( cmbStyle )->setCurrentIndex( cmbStyle->findText( name, Qt::MatchFixedString ) );
 
   QString theme = mSettings->value( QStringLiteral( "UI/UITheme" ), QStringLiteral( "default" ) ).toString();
+  if ( !QgsApplication::uiThemes().contains( theme ) )
+  {
+    theme = QStringLiteral( "default" );
+  }
   whileBlocking( cmbUITheme )->setCurrentIndex( cmbUITheme->findText( theme, Qt::MatchFixedString ) );
 
   mNativeColorDialogsChkBx->setChecked( mSettings->value( QStringLiteral( "/qgis/native_color_dialogs" ), false ).toBool() );