fix vue warning for undefined property during render

currentOs is not recognized as reactive property as it's set to "undefined". JavaScript does not accept "undefined" as valid value to initialize. A property needs to be initialized with a non-undefined value to become reactive in a class-based component. Otherwise Vue warns: Property or method "currentOs" is not defined on the instance but referenced during render.

src/presentation/components/Scripts/Menu/TheOsChanger.vue

@@ -24,7 +24,7 @@ import { ApplicationFactory } from '@/application/ApplicationFactory';
 @Component
 export default class TheOsChanger extends StatefulVue {
   public allOses: Array<{ name: string, os: OperatingSystem }> = [];
-  public currentOs?: OperatingSystem = undefined;
+  public currentOs?: OperatingSystem = null;
 
   public async created() {
     const app = await ApplicationFactory.Current.getAppAsync();