fix: broken 'set exchange gain/loss' btn in payment entry (#34940)

fix: broken set exchagne gain/loss btn broken in payment entry

(cherry picked from commit df0682f)

Co-authored-by: ruthra kumar <ruthra@erpnext.com>

erpnext/accounts/doctype/payment_entry/payment_entry.js

@@ -978,6 +978,7 @@ frappe.ui.form.on('Payment Entry', {
 							precision("difference_amount"));
 
 						const add_deductions = (details) => {
+							let row = null;
 							if (!write_off_row.length && difference_amount) {
 								row = frm.add_child("deductions");
 								row.account = details[account];