Fix popup closure after hitting the "escape" key

Steps to reproduce on the new WEB UI:

• click on the "search" icon on the left so that a listing of documents is displayed
• click on the "edit columns" button to open the popup (see sceenshot attached)
• hit the "esc" key to close the popup

=> it does not work ok and "freezes" the popup: clicking on the "close" button does not have any effect

Note: using FF 55.0.2 (64-bit) on linux