Add default value for BuildOrder reference

InvenTree/build/migrations/0030_alter_build_reference.py

@@ -0,0 +1,20 @@
+# Generated by Django 3.2.4 on 2021-07-08 14:14
+
+import InvenTree.validators
+import build.models
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('build', '0029_auto_20210601_1525'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='build',
+            name='reference',
+            field=models.CharField(default=build.models.get_next_build_number, help_text='Build Order Reference', max_length=64, unique=True, validators=[InvenTree.validators.validate_build_order_reference], verbose_name='Reference'),
+        ),
+    ]