Object referance error when change OrderQty in Sales Orders

I have a very strange problem with Modern UI. Here are the steps I take to get it: I create a sales order, set a kit, explode it, then without saving it, I create a new line with any item. When I try to set a QTY, it throws an object referencing error and role back qty to 0 on the tab and resets the QTY back to zero. This problem is related to Modern UI because Classic UI doesn't have this problem.

protected virtual void SOLine_OrderQty_FieldUpdated(PXCache sender, PXFieldUpdatedEventArgs e, PXFieldUpdated baseMethod)
{
    baseMethod?.Invoke(sender, e);

    SOOrderEntry soGraph = (SOOrderEntry)sender.Graph;

    SOLine row = (SOLine)e.Row;
    if (row == null)
        return;

    BZSOLineExt rowExt = PXCache<SOLine>.GetExtension<BZSOLineExt>(row);
    if (rowExt == null)
        return;

    switch (rowExt.UsrBZExplodeState)
    {
        case null:
            return;
        case BZExplodeState.Never:
            return;
    }

    if (soGraph.IsContractBasedAPI || row.InventoryID == null || (decimal?)e.OldValue == row.OrderQty)
        return;

    if (sender.Graph.Accessinfo.ScreenID != BZKitMessages.SOShipmentScreenId && sender.Graph.Accessinfo.ScreenID != BZKitMessages.OpportunityScreenId && sender.Graph.Accessinfo.ScreenID != BZKitMessages.QuoteScreenId)
    {
        if (rowExt.UsrBZMarkForKitAssembly == true && rowExt.UsrBZKitAssemblyRefNbr == null && rowExt.UsrBZKitAssemblyDocType == null)
            return;

        BZSOSetupExt setupExt = PXCache<SOSetup>.GetExtension<BZSOSetupExt>(soGraph.sosetup.Current);
        INKitSpecHdr hdr = GetKitItemSpecHeader(soGraph, row.InventoryID);
        if (hdr != null)
        {
            if (rowExt.UsrBZIsReplacedKit != true && row.IsKit == true)
            {
                BZSOOrderEntryExt soGraphExt = soGraph.GetExtension<BZSOOrderEntryExt>();

                if (!(soGraphExt.ExplodeFromPopupButton == true))
                {
                    CheckForKitExplosion(sender, row, rowExt, setupExt, GetItemByID(sender.Graph, row.InventoryID), GetExplodeOption(setupExt, hdr));
                }
            }
        }
    }
}

There's something strange: if you comment out this code, there won't be a problem; it seems to trigger the problem when Modern UI is enabled.