Can not save a fieldset

• Type: Impedibug
• Status: Closed
• Priority: Minor
• Resolution: Discarded
• Affects Version/s: None
• Fix Version/s: None
• Component/s: Data Engine
• Labels: None
• Sprint: Data_Engine_7.4_05, Data_Engine_7.4_06, Data_Engine_7.4_07

Steps to reproduce:

1. Go to Content&Data > Web Content > Strcuture
2. Navigate to FieldSets
3. Click Create New Fieldset
4. Add a fieldset title and a text field
5. Save the fieldset

Actual result:
Can not add a fieldset. there's an error in page:

Error:An unexpected error occurred.

Expected result:
Can add a fieldset.