Cannot edit activities with custom fields in popup from a search results screen

There appears to be an AJAX error when trying to edit activities with custom fields from the Find Activities search results screen. I have attached a .gif replicating the issue on the d46.demo.civicrm.org site.

Steps:

1. Created custom activity and associated custom fields (in the example the custom field is labelled 'something' with 4 options; option 1, option 2, etc).
2. Created an activity from the above
3. Search > Find activities > Filtered to find above custom activity
4. Click View on activity
5. Click edit on modal window
6. Attempt to change the value for the custom field
7. Receive validation error and value for the custom field has returned to the original value

Other notes:

• This does not happen when clicking edit from the search results screen (instead of view at step 4 above)
• The issue only occurs when there are required custom fields
• If there are multiple required custom fields, the error will list all fields and return the changed values to the original value
• I do not have instances below 4.6.10 to test this on so I am unsure if it affects previous versions.