After upgrading to Ubuntu 18.10 I cannot use enpass helper anymore, because it will close right after I click on it or alt+tab to it (or if it gains focus in any other way). This issue is happening for me on KDE Plasma (version 5.13.5). It was working fine before update (on Plasma 5.12.4).
If you're using KDE, here is quick workaround that works for me:
Go to System Settings -> Window Management -> Window Rules
Add new rule defined as on screenshots:
Save new rule
Click apply on settings page
Test if now enpass helper is going away after clicking on it (it shouldn't). You can test it either in browser or by clicking on enpass icon on system tray