Fix bug with removing attributes (#4927)

CHANGELOG.md

@@ -60,6 +60,7 @@ Skeleton (<https://github.com/cvat-ai/cvat/pull/1>), (<https://github.com/opencv
 - Fixed project filtration (<https://github.com/opencv/cvat/pull/4878>)
 - Maximum callstack exceed when create task with 100000+ files from cloud storage (<https://github.com/opencv/cvat/pull/4836>)
 - Fixed invocation of serverless functions (<https://github.com/opencv/cvat/pull/4907>)
+- Removing label attributes (<https://github.com/opencv/cvat/pull/4927>)
 
 ### Security
 - TDB

cvat/apps/engine/migrations/0060_alter_label_parent.py

+# Generated by Django 3.2.15 on 2022-09-09 09:00
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('engine', '0059_labeledshape_outside'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='label',
+            name='parent',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='sublabels', to='engine.label'),
+        ),
+    ]

cvat/apps/engine/models.py

@@ -498,7 +498,7 @@ class Label(models.Model):
     name = SafeCharField(max_length=64)
     color = models.CharField(default='', max_length=8)
     type = models.CharField(max_length=32, null=True, choices=LabelType.choices(), default=LabelType.ANY)
-    parent = models.ForeignKey('self', on_delete=models.CASCADE, null=True, related_name='sublabels')
+    parent = models.ForeignKey('self', on_delete=models.CASCADE, null=True, blank=True, related_name='sublabels')
 
     def __str__(self):
         return self.name