Disabling 802.1Q mode on interfaces don't clear untagged vlan assignment

[miaow2]

Environment

• Python version: 3.6.8
• NetBox version: 2.9.4

Steps to Reproduce

1. set on interface (on device or vm) 802.1Q Mode (any mode) and Untagged VLAN
2. set 802.1Q Mode to None, fields with vlans disapear
3. set any 802.1Q Mode again, field Untagged VLAN will be filled with VLAN from step 1

If you save after step 2, 802.1Q Mode will change, but Untagged VLAN stays on interface.

Expected Behavior

Untagged VLAN field should be empty

Observed Behavior

Untagged VLAN field does not clear, with Tagged VLANs field all works fine.

On my enviroment I fix this in file project-static/js/forms.js line 351 by sending empty string to val() function.

$('select#id_untagged_vlan').val('');

I don't know is this a good solution, so I open this issue

[jeremystretch]

This line seems to be the problem. Since we moved to slugs for choice fields, the mode field now stores an empty string rather than null.