"Add Sub-Interface" button disabled/faded when creating Custom Traffic Disaggregation

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Environment:
OS name: Ubuntu
OS version: 22.04.4 LTS
Architecture: x86_64
ntopng version/revision: 6.5.251114 (Enterprise XL)

What happened:
The "Add Sub-Interface" button remains disabled (faded/grayed out) when attempting to create a Custom Traffic Disaggregation by manually entering both the "Sub-Interface Name" and "Traffic Filter" fields. This prevents users from creating custom sub-interfaces through the normal GUI workflow.The button remains disabled regardless of:

• What filter syntax is entered (even when using the exact examples provided in the form)
• Whether the Sub-Interface Name field is populated
• When attempting to edit existing custom interfaces

How did you reproduce it?
Navigate to the Custom Traffic Disaggregation interface creation form
Enter a value in the "Sub-Interface Name" field (e.g., "test")
Enter a valid traffic filter in the "Traffic Filter (Extended nBPF Format)" field (e.g., EXPORTER_IPV4_ADDRESS '127.0.0.1')
Observe that the "Add Sub-Interface" button remains faded/disabled
Attempt to click the button - no action occurs

Workaround:
The action can be completed by using keyboard navigation:
Use the TAB key multiple times to navigate through the form elements
When the "Add Sub-Interface" button receives focus
Press ENTER to submit the form
The sub-interface is successfully created