[CLNP-6698] resetWithStartingPoint not defined error (#1332)

Fixes [CLNP-6698](https://sendbird.atlassian.net/browse/CLNP-6698)

### Changelogs

- Fixed a bug that `GroupChannelProvider` with `startingPoint` throws an
error

### Checklist

Put an `x` in the boxes that apply. You can also fill these out after
creating the PR. If unsure, ask the members.
This is a reminder of what we look for before merging your code.

- [x] **All tests pass locally with my changes**
- [ ] **I have added tests that prove my fix is effective or that my
feature works**
- [ ] **Public components / utils / props are appropriately exported**
- [ ] I have added necessary documentation (if appropriate)

[CLNP-6698]:
https://sendbird.atlassian.net/browse/CLNP-6698?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

---------

Co-authored-by: junyoung.lim <junyoung.lim@sendbird.com>

Author: chrisallo

src/modules/GroupChannel/context/GroupChannelProvider.tsx

@@ -248,10 +248,10 @@ const GroupChannelManager :React.FC<React.PropsWithChildren<GroupChannelProvider
 
   // Starting point handling
   useEffect(() => {
-    if (typeof startingPoint === 'number') {
+    if (typeof startingPoint === 'number' && messageDataSource.initialized) {
       actions.scrollToMessage(startingPoint, 0, false, false);
     }
-  }, [startingPoint]);
+  }, [messageDataSource.initialized, startingPoint]);
 
   // Animated message handling
   useEffect(() => {

src/modules/GroupChannel/context/__tests__/useGroupChannel.spec.tsx

@@ -304,6 +304,7 @@ describe('useGroupChannel', () => {
         const mockResetWithStartingPoint = jest.fn().mockResolvedValue(undefined);
         const mockStore = createMockStore({
           messages: [],
+          initialized: true,
           scrollRef: {
             current: document.createElement('div'),
           },

src/modules/GroupChannel/context/hooks/useGroupChannel.ts

@@ -115,7 +115,7 @@ export const useGroupChannel = () => {
       const topOffset = getMessageTopOffset(message.createdAt);
       if (topOffset) state.scrollPubSub.publish('scroll', { top: topOffset, animated: scrollAnimated });
       if (messageFocusAnimated ?? true) setAnimatedMessageId(messageId);
-    } else {
+    } else if (state.initialized) {
       await state.resetWithStartingPoint(createdAt);
       setTimeout(() => {
         const topOffset = getMessageTopOffset(createdAt);
@@ -130,7 +130,12 @@ export const useGroupChannel = () => {
       });
     }
     clickHandler.activate();
-  }, [setAnimatedMessageId, state.scrollRef.current, state.messages?.map(it => it?.messageId)]);
+  }, [
+    setAnimatedMessageId,
+    state.initialized,
+    state.scrollRef.current,
+    state.messages?.map(it => it?.messageId),
+  ]);
 
   const toggleReaction = useCallback((message: SendableMessageType, emojiKey: string, isReacted: boolean) => {
     if (!state.currentChannel) return;