[BUG] AttributeError: 'PurchaseOrderLineItem' object has no attribute 'purchase_price'
**Describe the bug**
Migration crashes with the following:
```
 Applying stock.0064_auto_20210621_1724...Found 1060 stock items with missing purchase price information
Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/home/inventree/.local/lib/python3.8/site-packages/django/core/management/__init__.py", line 419, in execute_from_command_line
    utility.execute()
  File "/home/inventree/.local/lib/python3.8/site-packages/django/core/management/__init__.py", line 413, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/inventree/.local/lib/python3.8/site-packages/django/core/management/base.py", line 354, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/inventree/.local/lib/python3.8/site-packages/django/core/management/base.py", line 398, in execute
    output = self.handle(*args, **options)
  File "/home/inventree/.local/lib/python3.8/site-packages/django/core/management/base.py", line 89, in wrapped
    res = handle_func(*args, **kwargs)
  File "/home/inventree/.local/lib/python3.8/site-packages/django/core/management/commands/migrate.py", line 244, in handle
    post_migrate_state = executor.migrate(
  File "/home/inventree/.local/lib/python3.8/site-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/home/inventree/.local/lib/python3.8/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/home/inventree/.local/lib/python3.8/site-packages/django/db/migrations/executor.py", line 227, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/home/inventree/.local/lib/python3.8/site-packages/django/db/migrations/migration.py", line 126, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/home/inventree/.local/lib/python3.8/site-packages/django/db/migrations/operations/special.py", line 190, in database_forwards
    self.code(from_state.apps, schema_editor)
  File "/home/inventree/InvenTree/stock/migrations/0064_auto_20210621_1724.py", line 46, in extract_purchase_price
    if line.purchase_price is not None:
AttributeError: 'PurchaseOrderLineItem' object has no attribute 'purchase_price'
```

I am not sure where the cause of this is coming from, I assume the migration `0064_auto_20210621_1724` is missing a dependency but I am not sure where what that would be.

**Steps to Reproduce**

Steps to reproduce the behavior:
I am upgrading to master on my production version of inventree.

**Expected behavior**
Migrate complete without crash.

**Deployment Method**
- [x] k8s
- [ ] Docker
- [ ] Bare Metal

**Version Information**
```
commit 844a537ec35e7d6ff2b685c6da64e72b1f33d61c (HEAD -> feature/maintdefaultstorage, nwns/feature/maintdefaultstorage)
Author: Nigel <nigel.w@nosun.ca>
Date:   Tue Feb 1 10:32:35 2022 -0700

    fix: use default storage backend for Maint Mode

commit 3462cf796baf70a1fc678827cb9d1c66868e53f7 (github/master, master)
Author: Oliver <oliver.henry.walters@gmail.com>
Date:   Sun Jan 30 21:44:23 2022 +1100```
diff --git a/InvenTree/stock/migrations/0064_auto_20210621_1724.py b/InvenTree/stock/migrations/0064_auto_20210621_1724.py
index 361ad02c8..666de2618 100644
--- a/InvenTree/stock/migrations/0064_auto_20210621_1724.py
+++ b/InvenTree/stock/migrations/0064_auto_20210621_1724.py
@@ -43,7 +43,7 @@ def extract_purchase_price(apps, schema_editor):
         if lines.exists():
 
             for line in lines:
-                if line.purchase_price is not None:
+                if getattr(line, 'purchase_price', None) is not None:
 
                     # Copy pricing information across
                     item.purchase_price = line.purchase_price
['InvenTree/stock/migrations/0064_auto_20210621_1724.py']