fix: attribute error when trying to fetch items (v11)

[Alchez]

Problem:

Frappe does not build tables as document attributes until it's actually processed first, causing the Production Plan to error out for new Plans.

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Tracebacks:

Sales Orders:

Traceback (most recent call last):
  File "/home/rohan/weed/apps/erpnext/erpnext/manufacturing/doctype/production_plan/production_plan.py", line 95, in get_items
    self.get_so_items()
  File "/home/rohan/weed/apps/erpnext/erpnext/manufacturing/doctype/production_plan/production_plan.py", line 100, in get_so_items
    so_list = [d.sales_order for d in self.sales_orders if d.sales_order]
AttributeError: 'ProductionPlan' object has no attribute 'sales_orders'

Material Requests

Traceback (most recent call last):
  File "/home/rohan/weed/apps/erpnext/erpnext/manufacturing/doctype/production_plan/production_plan.py", line 97, in get_items
    self.get_mr_items()
  File "/home/rohan/weed/apps/erpnext/erpnext/manufacturing/doctype/production_plan/production_plan.py", line 135, in get_mr_items
    mr_list = [d.material_request for d in self.material_requests if d.material_request]
AttributeError: 'ProductionPlan' object has no attribute 'material_requests'