Check to ensure theme setting is preset before setting electron nativ…

…e theme (#2890)

Fixes #2888

When setting the Electron nativeTheme with the theme selected in the Simplenote settings it can cause a JavaScript error if the theme setting isn't present, such as when logging out of the app.

This checks to ensure the theme setting is set before updating the Electron nativeTheme setting.

desktop/app.js

@@ -103,7 +103,9 @@ module.exports = function main() {
       Menu.setApplicationMenu(
         Menu.buildFromTemplate(createMenuTemplate(args), mainWindow)
       );
-      nativeTheme.themeSource = settings.theme;
+      if ('theme' in settings) {
+        nativeTheme.themeSource = settings.theme;
+      }
     });
 
     ipcMain.on('clearCookies', function () {