fix: add mergedData length check (#306)

* fix: add mergedData length check

* chore: check first

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Co-authored-by: 二货机器人 <smith3816@gmail.com>

Author: hakankosdag

src/List.tsx

@@ -279,13 +279,17 @@ export function RawList<T>(props: ListProps<T>, ref: React.Ref<ListRef>) {
     const changedRecord = heights.getRecord();
     if (changedRecord.size === 1) {
       const recordKey = Array.from(changedRecord)[0];
-      const startIndexKey = getKey(mergedData[start]);
-      if (startIndexKey === recordKey) {
-        const realStartHeight = heights.get(recordKey);
-        const diffHeight = realStartHeight - itemHeight;
-        syncScrollTop((ori) => {
-          return ori + diffHeight;
-        });
+      // Quick switch data may cause `start` not in `mergedData` anymore
+      const startItem = mergedData[start];
+      if (startItem) {
+        const startIndexKey = getKey(startItem);
+        if (startIndexKey === recordKey) {
+          const realStartHeight = heights.get(recordKey);
+          const diffHeight = realStartHeight - itemHeight;
+          syncScrollTop((ori) => {
+            return ori + diffHeight;
+          });
+        }
       }
     }