QGIS » QGIS Application

Hi,

While playing with constraints in the Attributes Form tab in a vector layer properties, I have found that once any constraint expression (valid or not) is saved, it is impossible to clear the constraint expression.
How to reproduce (tested on shapefile and postgis layers):

1. create a vector shapefile layer with a dummy field
2. go to the layer properties => Attributes Form
3. select the dummy field
4. type any random constraint expression (valid or not)
5. if your clear the input box before hitting apply, the expression will be empty as expected
6. if you hit apply, then any attempt to clear the input box will cause it to display its last saved value when the input lose focus

It seems that there is no way to clear it (aside deleting the layer); if I open the expression dialog it does not allow blank expression and if I just write a space it will be saved but it's an invalid expression thus I will have a warning when I edit a feature.