The code is as follows:

 protected void removeAllCells() {
        for (HSSFCell cell : cells) {
            if (cell != null) {
                removeCell(cell, true);
            }
        }
        cells=new HSSFCell[INITIAL_CAPACITY];
    }

NPOI fixed a bug:
https://github.com/nissl-lab/npoi/commit/7a885c42e2f21147c1c4b744db55f6d4af1e69fc

The buggy code is as follows:

public void RemoveAllCells(){
 int initialLen = cells.Count;
 for (int i = 0; i < initialLen; i++){
   RemoveCell(cells[i], true);
}}

The fixed code is as follows:
public void RemoveAllCells(){
  foreach (ICell cell in cells.Values){
    RemoveCell(cell, true);
}}