[web] Instantiate the issues proxy earlier

- For some reason, creating a proxy for issues fails.
- This same error has already been detected with other proxies, for
  example for the storage manager interface.
- Anticipating the creation of the proxies fixes the issue.
- Further research is needed to understand what happens at cockpit
  level.

web/src/client/mixins.js

@@ -75,13 +75,19 @@ const buildIssue = (dbusIssue) => {
  * @template {!WithDBusClient} T
  */
 const WithIssues = (superclass, object_path) => class extends superclass {
+  constructor(...args) {
+    super(...args);
+    this.proxies ||= {};
+    this.proxies.issues = this.client.proxy(ISSUES_IFACE, object_path);
+  }
+
   /**
    * Returns the issues
    *
    * @return {Promise<Issue[]>}
    */
   async getIssues() {
-    const proxy = await this.client.proxy(ISSUES_IFACE, object_path);
+    const proxy = await this.proxies.issues;
     return proxy.All.map(buildIssue);
   }