Check if devtools hook on/off methods are defined before using them in AppContainer (#39808)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39808

I've been doing some tests where I disable React DevTools locally and this code has been crashing because React Fast Refresh defines the React DevTools hook if DevTools didn't ([pointer](https://github.com/facebook/react/blob/85c2b519b54269811002d26f4f711809ef68f123/packages/react-refresh/src/ReactFreshRuntime.js#L453-L475)), and it doesn't actually define the `on` and `off` methods, which makes this throw.

This changes `AppContainer` to check if the methods are defined before using them.

Changelog: [internal]

Reviewed By: hoxyq

Differential Revision: D49909258

fbshipit-source-id: 3b0c1f37601a1cbe38f80ca3a20ffe2348a21aaf

packages/react-native/Libraries/ReactNative/AppContainer.js

@@ -94,10 +94,12 @@ class AppContainer extends React.Component<Props, State> {
           this._reactDevToolsAgentListener = () =>
             this.mountReactDevToolsOverlays();
 
-          reactDevToolsHook.on(
-            'react-devtools',
-            this._reactDevToolsAgentListener,
-          );
+          if (reactDevToolsHook.on != null) {
+            reactDevToolsHook.on(
+              'react-devtools',
+              this._reactDevToolsAgentListener,
+            );
+          }
         }
       }
     }
@@ -108,7 +110,10 @@ class AppContainer extends React.Component<Props, State> {
       this._subscription.remove();
     }
 
-    if (reactDevToolsHook != null && this._reactDevToolsAgentListener != null) {
+    if (
+      reactDevToolsHook?.off != null &&
+      this._reactDevToolsAgentListener != null
+    ) {
       reactDevToolsHook.off('react-devtools', this._reactDevToolsAgentListener);
     }
   }