Fixed an "Uncaught ReferenceError" inside of the @astrojs/vue package (…

…#12116)

* fix(vue): initialized the 'app' variable earlier in the client.js script by assigning the 'appInstance' reference to it so that it is available for the later established 'astro:unmount' event listener

* Add changeset

* fix(vue): reverted the last change and instead moved the 'astro:unmount' event listener to inside of the 'if' statement where the 'app' variable is initialized

---------

Co-authored-by: Matt Kane <m@mk.gg>

.changeset/wicked-sloths-cry.md

@@ -0,0 +1,5 @@
+---
+"@astrojs/vue": patch
+---
+
+Fixes an Reference Error that occurred during client transitions

packages/integrations/vue/client.js

@@ -44,12 +44,12 @@ export default (element) =>
 			await setup(app);
 			app.mount(element, isHydrate);
 			appMap.set(element, appInstance);
+			element.addEventListener('astro:unmount', () => app.unmount(), { once: true });
 		} else {
 			appInstance.props = props;
 			appInstance.slots = slots;
 			appInstance.component.$forceUpdate();
 		}
-		element.addEventListener('astro:unmount', () => app.unmount(), { once: true });
 	};
 
 function isAsync(fn) {