fix(xo-server/proxy.upgradeAppliance): use getObject method on correct object

Introduced by 572359892

CHANGELOG.unreleased.md

@@ -12,6 +12,7 @@
 > Users must be able to say: “I had this issue, happy to know it's fixed”
 
 - [Dashboard/Health] Fix `Unknown SR` and `Unknown VDI` in Unhealthy VDIs (PR [#6519](https://github.com/vatesfr/xen-orchestra/pull/6519))
+- [Proxies] Fix `this.getObject is not a function` on upgrade
 
 ### Packages to release
 

packages/xo-server/src/xo-mixins/proxies.mjs

@@ -200,7 +200,7 @@ export default class Proxy {
     const { vmUuid } = await this._getProxy(id)
     if (vmUuid !== undefined) {
       try {
-        this.getObject(vmUuid, 'VM')
+        this._app.getObject(vmUuid, 'VM')
 
         isVmKnown = true
       } catch (error) {