fix: handle scrollRef.current == null case (#516)

### Description Of Changes
Resolves [UIKIT-3790](https://sendbird.atlassian.net/browse/UIKIT-3790)

scrollRef.current could be null and it caused a runtime error. So added a safe guard for it.

Author: AhyoungRyu

src/hooks/useHandleOnScrollCallback/index.ts

@@ -19,7 +19,11 @@ export function useHandleOnScrollCallback({
   setShowScrollDownButton,
 }: UseHandleOnScrollCallbackProps): () => void {
   return useCallback(() => {
-    const element = scrollRef.current;
+    const element = scrollRef?.current;
+    if (element == null) {
+      return;
+    }
+
     const {
       scrollTop,
       scrollHeight,
@@ -40,7 +44,7 @@ export function useHandleOnScrollCallback({
     if (scrollTop < SCROLL_BUFFER) {
       onScroll(() => {
         // sets the scroll position to the bottom of the new messages
-        scrollRef.current.scrollTop = scrollRef.current.scrollHeight - scrollBottom;
+        element.scrollTop = element.scrollHeight - scrollBottom;
       });
     }
   }, [

src/modules/Channel/components/MessageList/index.tsx

@@ -58,8 +58,12 @@ const MessageList: React.FC<MessageListProps> = ({
     : allMessages;
   const markAsReadScheduler = store.config.markAsReadScheduler;
 
-  const onScroll = (e) => {
-    const element = e.target;
+  const onScroll = () => {
+    const element = scrollRef?.current;
+    if (element == null) {
+      return;
+    }
+
     const {
       scrollTop,
       clientHeight,
@@ -80,10 +84,7 @@ const MessageList: React.FC<MessageListProps> = ({
     }
 
     // Save the lastest scroll bottom value
-    if (scrollRef?.current) {
-      const current = scrollRef?.current;
-      setScrollBottom(current.scrollHeight - current.scrollTop - current.offsetHeight)
-    }
+    setScrollBottom(element.scrollHeight - element.scrollTop - element.offsetHeight)
 
     if (!disableMarkAsRead && isAboutSame(clientHeight + scrollTop, scrollHeight, SCROLL_BUFFER)) {
       messagesDispatcher({