FIX: more resilient lookup in plugin-api (#6961)

Ember3 is more picky about having a container being destroyed and it's easier to cause exceptions, especially in tests.

This fix has been initially created for an exception occuring in tests when running discourse-code-review and discourse-polls tests at the same time. `getCurrentUser` method body was failing as the container was destroyed.

Original  stacktrace:

```
"Error: Assertion Failed: expected container not to be destroyed
    at new EmberError (ember:2929:31)
    at assert (ember:1793:23)
    at Container.lookup (ember:17736:64)
    at PluginApi.getCurrentUser (discourse/lib/plugin-api:56:31)
    at allowUser (javascripts/discourse/initializers/init-code-review:38:29)
    at eval (javascripts/discourse/initializers/init-code-review:78:11)
    at eval (select-kit/mixins/plugin-api:86:19)
    at Array.forEach (<anonymous>)
    at eval (select-kit/mixins/plugin-api:85:44)
    at Array.forEach (<anonymous>)"
```

app/assets/javascripts/discourse/lib/plugin-api.js.es6

@@ -55,7 +55,19 @@ class PluginApi {
    * If the user is not logged in, it will be `null`.
    **/
   getCurrentUser() {
-    return this.container.lookup("current-user:main");
+    return this._lookupContainer("current-user:main");
+  }
+
+  _lookupContainer(path) {
+    if (
+      !this.container ||
+      this.container.isDestroying ||
+      this.container.isDestroyed
+    ) {
+      return;
+    }
+
+    return this.container.lookup(path);
   }
 
   _resolveClass(resolverName, opts) {
@@ -222,7 +234,7 @@ class PluginApi {
    * ```
    **/
   addPosterIcon(cb) {
-    const site = this.container.lookup("site:main");
+    const site = this._lookupContainer("site:main");
     const loc = site && site.mobileView ? "before" : "after";
 
     decorateWidget(`poster-name:${loc}`, dec => {
@@ -424,8 +436,8 @@ class PluginApi {
     ```
   **/
   onAppEvent(name, fn) {
-    let appEvents = this.container.lookup("app-events:main");
-    appEvents.on(name, fn);
+    const appEvents = this._lookupContainer("app-events:main");
+    appEvents && appEvents.on(name, fn);
   }
 
   /**
@@ -562,7 +574,8 @@ class PluginApi {
    * will issue a request to `/mice.json`
    **/
   addStorePluralization(thing, plural) {
-    this.container.lookup("service:store").addPluralization(thing, plural);
+    const store = this._lookupContainer("service:store");
+    store && store.addPluralization(thing, plural);
   }
 
   /**