[fix] Reference Document is required to create Payment Request

[saurabh6790]

---

Traceback (most recent call last):
  File "/Users/saurabh/frappe-bench/apps/frappe/frappe/desk/form/save.py", line 22, in savedocs
    doc.save()
  File "/Users/saurabh/frappe-bench/apps/frappe/frappe/model/document.py", line 231, in save
    return self._save(*args, **kwargs)
  File "/Users/saurabh/frappe-bench/apps/frappe/frappe/model/document.py", line 254, in _save
    self.insert()
  File "/Users/saurabh/frappe-bench/apps/frappe/frappe/model/document.py", line 193, in insert
    self.run_before_save_methods()
  File "/Users/saurabh/frappe-bench/apps/frappe/frappe/model/document.py", line 772, in run_before_save_methods
    self.run_method("validate")
  File "/Users/saurabh/frappe-bench/apps/frappe/frappe/model/document.py", line 667, in run_method
    out = Document.hook(fn)(self, *args, **kwargs)
  File "/Users/saurabh/frappe-bench/apps/frappe/frappe/model/document.py", line 892, in composer
    return composed(self, method, *args, **kwargs)
  File "/Users/saurabh/frappe-bench/apps/frappe/frappe/model/document.py", line 875, in runner
    add_to_return_value(self, fn(self, *args, **kwargs))
  File "/Users/saurabh/frappe-bench/apps/frappe/frappe/model/document.py", line 661, in 
    fn = lambda self, *args, **kwargs: getattr(self, method)(*args, **kwargs)
  File "/Users/saurabh/frappe-bench/apps/erpnext/erpnext/accounts/doctype/payment_request/payment_request.py", line 18, in validate
    self.validate_currency()
  File "/Users/saurabh/frappe-bench/apps/erpnext/erpnext/accounts/doctype/payment_request/payment_request.py", line 26, in validate_currency
    ref_doc = frappe.get_doc(self.reference_doctype, self.reference_name)
  File "/Users/saurabh/frappe-bench/apps/frappe/frappe/__init__.py", line 606, in get_doc
    return frappe.model.document.get_doc(arg1, arg2)
  File "/Users/saurabh/frappe-bench/apps/frappe/frappe/model/document.py", line 47, in get_doc
    doctype = arg1.get("doctype")
AttributeError: 'NoneType' object has no attribute 'get'