Fix form mode save button from attr dialog

[elpaso]

Fix #45527

[nyalldawson]

Is this a new regression? If not, I'd suggest we defer merging till after 3.22.0 (given the fragility of attribute table/form signal connections and the number of unintended side effects they cause)

[tudorbarascu]

To me it's a regression IMHO but I understand the fragility of the attribute table. As this should also be backported to 3.16, wouldn't it be better to try it in 3.22 until it reaches LTR status? I'm building the master with this patch anyhow so I'm going to be one of the testers for sure but it would be great if we could have wider testing.

[elpaso]

@tudorbarascu are you sure it's a regression? Did you check if this was working on 3.16?

[gioman]

@elpaso it was first reported by me here #37795 but I don't think it was a regression even then. I should check to be 100% sure.