Fixes: #5315 - Make "null_option" on DynamicModelChoiceField also null the value on the model.

docs/release-notes/version-2.10.md

@@ -1,5 +1,13 @@
 # NetBox v2.10
 
+## v2.10.5 (FUTURE)
+
+### Bug Fixes
+
+* [#5315](https://github.com/netbox-community/netbox/issues/5315) - Fix site unassignment from VLAN when using "None" option
+
+---
+
 ## v2.10.4 (2021-01-26)
 
 ### Enhancements

netbox/utilities/forms/fields.py

@@ -355,7 +355,11 @@ class DynamicModelChoiceField(DynamicModelChoiceMixin, forms.ModelChoiceField):
     Override get_bound_field() to avoid pre-populating field choices with a SQL query. The field will be
     rendered only with choices set via bound data. Choices are populated on-demand via the APISelect widget.
     """
-    pass
+
+    def clean(self, value):
+        if self.null_option is not None and value == 'null':
+            return None
+        return super().clean(value)
 
 
 class DynamicModelMultipleChoiceField(DynamicModelChoiceMixin, forms.ModelMultipleChoiceField):