update(zoom): set store instance before emitting pane-ready event

* add missing generics on getters
* replace ? notation with undefined (does not mean the exact same, i.e.
`instance?: FlowInstance` will become `Ref<FlowInstance | undefined> | undefined`
using `instance: FlowInstance | undefined` results in `Ref<FlowInstance | undefined>`

Signed-off-by: Braks <78412429+bcakmakoglu@users.noreply.github.com>

src/container/ZoomPane/ZoomPane.vue

@@ -205,9 +205,9 @@ onMounted(async () => {
     getEdges: onLoadGetEdges(store),
     toObject: onLoadToObject(store),
   }
-  store.hooks.paneReady.trigger(instance)
   store.instance = instance
   store.fitViewOnInit && instance.fitView()
+  store.hooks.paneReady.trigger(instance)
 })
 </script>
 <script lang="ts">