refactor: register plugin application context after context refreshed (…

…#4092)

#### What type of PR is this?
/kind improvement
/area core
/area plugin

#### What this PR does / why we need it:
插件的 ApplicationContext 刷新之后在注册到 Registry 中

#### Does this PR introduce a user-facing change?

```release-note
None
```

application/src/main/java/run/halo/app/plugin/PluginApplicationInitializer.java

@@ -95,7 +95,6 @@ private PluginApplicationContext createPluginApplicationContext(String pluginId)
         log.debug("Total millis: {} ms -> {}", stopWatch.getTotalTimeMillis(),
             stopWatch.prettyPrint());
 
-        contextRegistry.register(pluginId, pluginApplicationContext);
         return pluginApplicationContext;
     }
 
@@ -126,6 +125,8 @@ private void initApplicationContext(String pluginId) {
         pluginApplicationContext.refresh();
         stopWatch.stop();
 
+        contextRegistry.register(pluginId, pluginApplicationContext);
+
         log.debug("initApplicationContext total millis: {} ms -> {}",
             stopWatch.getTotalTimeMillis(), stopWatch.prettyPrint());
     }