Profile: Text in Page Size leads to Exception

Exception is shown after changing paze size drop-down into text field

Preconditions:
WebDev plugin should be installed
Steps to reproduce:

1. Go to Profile->Edit
2. Right click on "page size"-> web developer menu -> forms -> convert selected elements to inputs -> enter text "bla bla bla"
3. Press "save changes button"

Actual result: Exception is shown
Expected result: Error message about incorrent data is shown