Fixed number is NULL constraint.

dav_events/migrations/0005_auto_20180124_1725.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.9 on 2018-01-24 17:25
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('dav_events', '0004_auto_20180124_1650'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='event',
+            name='number',
+            field=models.CharField(blank=True, default=None, max_length=12, null=True, unique=True),
+        ),
+    ]