fix(index): allow dispose to be called before init

fixes #6073

packages/instantsearch.js/src/widgets/index/__tests__/index-test.ts

@@ -3155,6 +3155,16 @@ See documentation: https://www.algolia.com/doc/api-reference/widgets/index-widge
 
       expect(mainHelper.derivedHelpers).toHaveLength(0);
     });
+
+    it('does not crash when calling `dispose` before `init`', () => {
+      const instance = index({ indexName: 'indexName' });
+
+      instance.addWidgets([virtualSearchBox({})]);
+
+      expect(() => {
+        instance.dispose(createDisposeOptions());
+      }).not.toThrow();
+    });
   });
 
   describe('getWidgetState', () => {

packages/instantsearch.js/src/widgets/index/index.ts

@@ -758,16 +758,16 @@ const index = (widgetParams: IndexWidgetParams): IndexWidget => {
 
     dispose() {
       localWidgets.forEach((widget) => {
-        if (widget.dispose) {
+        if (widget.dispose && helper) {
           // The dispose function is always called once the instance is started
           // (it's an effect of `removeWidgets`). The index is initialized and
           // the Helper is available. We don't care about the return value of
           // `dispose` because the index is removed. We can't call `removeWidgets`
           // because we want to keep the widgets on the instance, to allow idempotent
           // operations on `add` & `remove`.
           widget.dispose({
-            helper: helper!,
-            state: helper!.state,
+            helper,
+            state: helper.state,
             parent: this,
           });
         }